I am unfortunately have too much familiarity with this situation. After 8-9 years of being together with no problems whatsoever, my husband started to experience exactly what you're describing. Half-mast, can orgasm (actually quite quickly) but difficulty getting fully erect.
He went to the doctor, there's nothing physically wrong with him. He got boner pills - they didn't work. Because the problem is anxiety, and those pills don't work for anxiety (except perhaps as a placebo).
The good news is, it's not permanent. When my husband's anxiety recedes, he's just as capable as ever at getting and maintaining erections, but if he fears it's not going to work, it won't work. Sometimes he'll start out very hard and as the anxiety spiral about "what if I lose it?" starts in his head away it goes. Even if he's desperate to have sex.
The worst thing you can do is to bring it up as an issue for you. He already feels the pressure, believe me. Knowing you are desperate to be fucked is not going to help, it's going to hurt. If you want to keep dating him, you need to do your best to remove expectations for PIV sex off the table for now - and tell him so. "Hey, I know it's been awkward since you've been having a hard time (lol pun) and I just want you to know that I enjoy our time together and I'm fine being intimate without penetration for now, OK? So don't worry about it, let's just have fun."
That won't solve it overnight, but at least you'll have taken pressure on your side off of him. Hopefully that will give him time to feel more relaxed and put less pressure on himself. He probably is having a hard time because he really likes you and doesn't want to fuck it up...which of course is counterproductive but the link between a man's anxiety and his erections is so strong, he's not in control of it.
Then give yourself an internal timeline for how long you're willing to wait, don't tell him, and see how it plays out. If after 2 months or 4 months or whatever your timeline is, things still aren't working, then break it off.
If you were in a more established relationship, I'd suggest asking him to cut down on masturbation/porn, you said he already works out, but something that also helped for my husband was meditation. Helps get him out of his own head. But you don't have an established relationship and at this point talking further (beyond telling him you don't expect PIV at this time) is probably going to do more harm than good. Trust him to try to figure it out on his own.