I almost always use an indoor rotisserie for my wild boar roasts. (it's awesome!) I don't cook roasts on the grill very often. On those rare times, I will wrap the roast in aluminum and cook it until done, then take it out of the foil, throw some woodchips on the fireand sear the roast in the flames.
I also marinateroasts a bit and then rub in spices right before cooking.
If you're hog doesn't have any fat, be sure to rub some oil on the roast before cooking it.
Like anything else don't cook it until it's dried up and hard.