J.O.
Between 2 and 2 1/2 is when I have always found it to be the easiest to potty train a child. I think once they start to get older they start to see the control they have over their bowels movements and it can make it a lot harder on the parents.
You need a good 3-4 day period to really buckle down the hatches and get him trained. Get rid of the pull-ups (except nap and bedtime). Get rid of the plastic liners. Have a mop on hand or a carpet cleaner on hand. I also find multiple potties throughout the house make it a whole lot easier.
Naked is best for the first day or so, this helps them associate the sensation with the effect. Then put on undies, expect an accident or two, they kind feel like diapers at first. Stay consistent and do not get mad at accidents. Accidents are normal, just point him in the right direction.
As for grandma, if she can't be consistent, then he doesn't go there till he is completely trained. You also need to get him to do some sitting to pee, this is when the pooping will happen.
Make a HUGE deal when he is successful.
Honestly right now is about training you. Then you will slowly see the reins handed over to him.
I have never had a kid in diapers past 2 1/2, and he had learning disabilities.
You can do it, you just have to really ready to make it happen.