Potty Training

The boy is 3 years and 2 months.  We finally started potty training.  It is not going well.  It is not interested at all.  He likes wearing underwear and he knows what is going on down there.  He just refuses to sit on the potty.  I let him pick out what potty he wanted.  He wanted the truck one.  But he just doesn't want to use it.  I'm not sure if I should worry about it now or not.  Our nephew didn't potty train until he was 3 1/2 so it's not like Bub is behind for his age.  Some of the kids in his class are potty trained and some are not.  I'm hoping seeing his friends will encourage him.  When he saw his cousin using the potty he wanted to too so I guess I shouldn't stress about it now.  If I stress and then stress him out it will take even longer.  I'm just so sick of changing diapers.