S.M.
I would have them go into the bathroom with ALL the other boys and dad so they can see what is going on in there. Have the older boys say positive things about going to the potty. ALso try throwing some cheerios in the toilet and have the boys "shoot" the cereal. My mom had to do this with my brother when he was younger and it works. My son couldn't be bothered when I tried to train until he saw my fiance going. Once he realized they had the same stuff, he wanted to be BIG boy like dad. I don't know if this will help, but at least it is something to try.