H.
My daughter is recently potty trained as well and we do have days where she has an accident or even two and it seems to be for no apparent reason. I think reenforcing that he is a big boy and to keep reminding him that he needs to go on the potty are necessary but the diaper may be too tramatic from the sounds of it...
Has anything big or trasitioning happened recently? a move, reletives in town, changes in routine, new baby... sometimes these little ones regress with major life events a little. Also my daughter just had a Urinary tract infection and I think it affected her ability to control her little bladder somewhat... so you could also have the dr. check him just to make sure there are no illnesses causing it.
Good luck