It's awesome that you care so much and want to help him-he is lucky to have you. Remember that you don't have the weight of all of this on your shoulders. The best way to help someone is to ask them what they need. Ask him! It's important to remember that he's got some heavy stuff going on, and you don't want to get too far into listening to him vent about the relationship, since it is your mom. It's also important to remember that this can cause some big emotions for you too. It's really cool that you want to help him, but don't put off your personal needs. This is impacting YOUR family, so you need to be able to talk about what emotions you are feeling. His relationship with your mom is independent of you, so if he gets angry or depressed remember that it is not your fault. Some things that could be good:
Suggest you guys do stuff that you have always done. If you both love fishing then make that a priority for a bit. Lots of stuff is changing for him, so it might be nice to have some routines still in effect.
Encourage him to take care of himself. Be physically active and pursue his interests. Did he used to motorcycle and sold it when you were born? Encourage him to buy one.
Don't let him vent to you about mom. If he starts, then let him know that it makes you uncomfortable talking about mom, and that you want to spend time with him focusing on the present. It's ok for him to say "I'm really hurt by everything that has happened" or "I'm really sad that this didn't work out." What's not ok is stuff like "If your mom hadn't divorced me we would be so happy" or "Is your mom seeing anyone? I always thought she was interested in our neighbor."