Hi T.! My son didn't start really having any success with potty training until he was 3, so don't worry...it takes boys longer to pick up on these things than girls. I don't know what you could do about the underwear being too small, but I can tell you how we got my little guy potty trained.
We put him in "underoos," and every hour we put him on the potty. You probably already know this, but it's best to have them straddle the potty instead of having their feet in front...that way they don't feel like they're going to fall in. So we would just have him sit there until he went. We made it fun by singing a silly song (Tee tee in the potty!). And we would just sing it until he went. He went at least a little bit every time, and he always got to high five us after washing his hands. We did that for about a week, and by Friday, he was good to go. His babysitter taught us how to do it, and it was so easy and effective.
As for your mom, you may want to just accept that she's going to have her opinion, but that doesn't mean she's right. Every kid is different, so don't feel like you're doing this wrong. As long as you're positive with him about it and make it fun, he'll get the hang of it. Good luck!