Im not a speech pathologist but I was a therapy assistant in an early intervention center run by my state's health department in Australia and worked very closely with Speech Paths implementing therapies so I might have some knowledge that i soaked up there that I think could help you.
The biggest tools you can use to help your son increase his language is by exposing him to more words and repeating them and giving him opportunities to use the words you are modeling for him. The times when he is going to be most receptive to learning it when he is invested in the activity. Like when it is a toy he wants to play with or if he likes going for drives in the car or talking about the snack he wants to eat.
I wrote out a whole novel but im just going to copy/paste what i found here Which does a much better job of explaining it all than I did!
When working with kids on language skills, your goal should always be to help them reach just the next level of complexity—nothing more and nothing less. For example if your child communicates in one or two word bursts, your goal should be to model and use three and four word sentences. But make sure to follow your child's lead so they remain engaged and empowered to try out new words and communicate in new ways. Talking and communicating with others should be fun! Remember, language is something kids acquire naturally, so we want to watch what they're interested in, observe and listen to how they communicate, and help bring them to the next level of language skill.
Here are some strategies you can use with kids from birth all the way up to five years old, depending upon their language level. How you do these things may look different during infancy compared to when kids are starting to use words, but the basic idea will remain the same.
Imitate: If your daughter is making noises (babbling), making another sound in play, or even banging a spoon, you can do that too. Imitating children's sounds, words, and actions shows them that they're being heard and that you approve of what they're doing or saying. It also promotes turn taking and, best of all, encourages them to imitate you and your more complex language utterances.
Interpret: If your son is pointing to the apple juice that he wants to drink, he is communicating with you. Take this to the next level by interpreting what he is trying to say. Respond with, "Apple juice! You want apple juice!"
Expanding and recasting: When your daughter says "red truck," you can expand on that by saying, "Yes, a big red truck." If your son says, "The dragon jumping on the bed," you can recast his grammar by saying, "The dragon is jumping on the bed. Use stress and intonation to highlight the words you want your child to focus on.
Commenting and describing: Instead of telling kids what to do during playtime, be a sportscaster and give a play-by-play of what they're doing. Say, "You're driving the red car around in circles," or, "You're putting the cow into the barn. The cow is going to sleep." This models good vocabulary and grammar and helps kids organize their thoughts. Maybe they weren't actually putting the cow to sleep—maybe they were just putting it inside the barn—but by suggesting that you've given them a new concept to consider.
Eliminate negative talk: Try not to say things like, "That's not where the cow goes," or, when they're coloring, "The sky isn't pink." Remember we want to encourage all attempts to communicate and validate those attempts so that kids do more of it. We all respond better to more positive phrasing.
Contingent responses: Respond immediately to all attempts to communicate, including words and gestures. This is a big one. It shows kids how important communication is and gives you the opportunity to model more sophisticated language skills.
Balance turn taking: Give kids the space to exercise their communication skills by making sure they get a turn. Turns don't need to be talking, either. A turn could be your child handing you a toy or making eye contact. Maybe your daughter will look at you because she needs help opening a box. You can say, "You need help opening the box!" Then you can wait for her to hand you the box—that's her taking another turn. Turn taking can be hard for parents because we're used to taking charge of situations, but it is important to give kids the opportunity to use the skills they are developing.
Label things: Even when kids aren't ready to use words yet, you can prepare them by labeling things in their environment. During bubble baths keep referring to the bubbles; during snack time you can label the apple juice.
Limit "testing": If you know that your son knows which sound a pig makes, don't keep asking him. Testing him during playtime instead of just playing with him can be stressful. Instead you could say, "I wonder where the pig is going?" It still invites him to respond, but it doesn't put him on the spot.
Labeled praise: Instead of just saying "good job," put a label on that praise. If you're child isn't yet using words, (or even if they are) you could say, "Good job putting all the blocks back," because it reinforces their good behavior even more. For a child who is using some words to communicate, you could say, "Nice job telling me that you want apple juice," or "Nice job saying more juice please." This will help create positive feelings around communication and motivate them to continue to try and add new words.
TLDR: Talk to him as much as you can and engage in what he is interested in. Get down on his level and give him opportunities to use the language you're modeling for him.