Keeping son dry in underwear

Hi IMs,

My almost 3 yr old son has been learning to use the toilet for a while now. He is great at doing poos on the toilet (will even take his nappy off to go) which we are really happy with as we know that's normally the hardest part! He will do wees on the toilet if he has no nappy or underwear on, but every time we try and put him in underwear, he wets himself. At this point, I find it easier to leave him naked from the waist down when at home and put him in a nappy when we go out, but we can't keep doing that forever!! Any advice on how to keep him dry in underwear would be greatly appreciated! TIA