Girl,
Go with it. Just follow his lead. When it's about time for him to go... we sat our son about 15 - 20 mins after each meal. This didn't always work, but it established a routine for him.
Since you're at home with him and your daughter, I would suggest whenever she goes, have him go to. This will help reinforce the potty training. Another thing that helped when we were training our son was taking him to sit on his potty whenever we were going to go.
Since we put our son into big boy underwear right away, he had a few accidents, but we just dealt with the messes. His daycare didn't care as long as they had enough changes for him. From the time he started showing interest at 18 months; he was completely trained by 2yrs 7mos.
So stick with it and establish a routine as soon as you can with him. Having your daughter as a role model for him will help too.
Good luck!!!
M. H