Celebrate that your son is showing signs of readiness, and RUN WITH IT! In this day and age, so many parents are ignoring signs of readiness because they've been fed the "boys potty train late" line or the "kids aren't ready until at least 2 1/2 yrs old" myth. Or they delay and delay because they want potty training to be convenient and accident-free from day one, so they ignore signs of readiness because any accidents signal to them "not yet ready"... but then they wonder why their 3 yr old only wants to poop in the diaper. Well, because that's what we've trained them to do all of these years, and then we've even ignored their previous signs of readiness! Don't fall for it - your son shows any interest? Dive right in.
Potty training is like learning to walk - there will be falls, and that's how they learn: when they're given many chances to try. So, I encourage you to start potty training him and thus honor/respect your son's interest and readiness signs. He is NOT too young at all. :) Just keep your expectations realistic (think weeks & months, not days), expect some accidents (again, these are great learning opportunities), remain positive and supportive and relaxed, and never punish or shame.
As far as refusing to sit on the potty - you have several options here:
* make potty time a family affair, like you all go to the bathroom together and mom or dad sits on the adult toilet (you model the behavior for him) while he sits on a little plastic potty (he imitates you) and keep an open dialogue going about where pee goes, what it feels like when you have to pee (so he can connect the feeling to the outcome, no pun intended)
* running a trickle of water in the sink or the sound of the parent peeing will often encourage your son to release/relax his sphincter and pee soon thereafter (another advantage of family pottying)
* offer an interesting book or toy on which he can focus his attention while sitting on the potty - often this will allow him to relax his body enough and release the pee - and give him ample time
* if he absolutely does not want to sit, well, help him figure out how to stand and pee - stick him in the shower/bathtub at first until he gets use to releasing the sphincter/peeing and then gradually transition him to the adult toilet (provide him with a step stool to stand on and help him learn how to aim into the toilet bowl), or buy a "wee man" kiddie urinal like this one: http://www.pottytrainingconcepts.com/Potty-Training-Urina...
Congratulations on recognizing your son's readiness signs! As long as you keep potty training a relaxed, positive affair that is considered a process (like learning to walk, stumbling along the way but getting more proficient and sturdy over time) instead of a quickie event, you will all be fine. And oh yeah, keep him out of diapers and in those cool, simple boxers or even bare-bummed as much as humanly possible, and I promise the potty training will go faster (even if initially there's more laundry to do, which by the way, he can help wash as long as it's done as a team effort/natural consequence and NOT negatively or as punishment). Allowing him to go bare-bummed or in simple boxers gives him the opportunity to really connect-the-dots very quickly so that he "gets it" that his body's pee/poop sensations result in pee/poop coming out. Absorbent, waterproof diapers and training pants (even those fancy "wetness liner" ones) don't help - they're just expensive, slow down the process, and send a mixed message to our kids about where to go to the bathroom.
Have fun & happy pottying!