M.B.
IF you can allow him to be w/o underwear or a diaper - that's the easiest start.
My son is now potty trained - he is 2 years and 8 months old. He doesnt wet his underwear at night either. I am sooo excited about his success.
We started by letting him be diaper/underwear free pretty much all day. We would ask him throughout the day if he had to go "pee-pee" (use whatever word you prefer) or "poopie" and he'd say yes or no. If my husband or I had to use the toilet we'd take him along with us and tell him what we were doing.
First thing's first: get him acquainted with either a potty or your toilet. Don't force him to sit on it because he'll get scared. My son thought it was funny when he'd sit on the potty and I'd be using the toilet and I'd make the #1 or #2 noises. He liked to see me become animated and this made him less afraid and anxious of his "duty" in the bathroom.
If he'd do #2 in his diaper, I'd put it in the toilet and wave bye bye to it saying "no more poopie in the diaper, poopie goes in the toilet now".
When he would take initiative to try and make it to the toilet in time, we'd praise him with High-Fives and kisses and hugs. He liked the praise. If he went a day without doing #1 on the floor or in his diaper, I'd tell him at the end of the day how great he did trying to use the toilet like a big boy.
There's a book, Potty Time (sold at Target, for sure) and it has a button that sounds like a flushing toilet - he likes it a lot (still). You can substitute your own words to make the story personal. Its simple and its for boys or girls - you choose the book.
Now, when he does #1, he gets onto the toilet seat - facing it and this is how he makes sure he doesnt wet the floor. He announces it and we say "hurry, quickly, let's go!" and he likes to go alone but he likes us to notice. As he's progressed we've given him stickers which he sticks to a piece of paper (he likes trains and Dora - so he chooses which one he wants) that I made for him to put onto his bedroom door. He has lots of stickers now and he enjoys seeing them as a sign of his accomplishments.
Good luck, I hope I did not write too much.