It wouldnt hurt to take a look at Jessica Seinfields cookbook called 'Deceptively Delicious' It will show you some ways to hide fruits and vegies in different foods. My kids love the brownies they have spinich leaves in them and I promise you cant even taste them, I think its great. Good Luck!!
Miralax works great to soften the stool, you stir it into juice or whatever you want. It's over the counter now, available at drug stores or walmart & target. I think the container says to just use 2 weeks but I know doctors prescribe it for much longer at a time so if you are concerned just call his pediatrician to confirm how long is okay.
I would ask the doctor and would definitly make sure he is getting vegetables at 3 yrs old he needs vegetables and fruit.