Hand, foot and mouth syndrome causes tiny blisterlike sores in the mouth, on palms of hands, and soles of feet. It's usually accompanied with mild fever, sore throat and painful swallowing lasting around 7 days. The incubation time is 3-6 days. Has he been around anyone else with this virus? It is very contagious and is just a yucky childhood virus. You should give him acetaminophen (Infant Tylenol) and push fluids. Popsicles are the best! It's frustrating for parents because there isn't much more that we can do to help them get through this miserable virus.
Avoid acidic juices like orange juice as that may burn his mouth and just keep offering smoothies and soft food. He'll eventually feel like eating again. And, I would definately keep him home! It amazes me how many parents take their sick kids out (to church, to Walmart, to school) and not only expose everyone else but negate the fact that just like when we are sick, we feel most comfortable at home, lounging in our pj's and resting.
Even the rash on his bottom could be making him uncomfortable. I would just try to wipe gently when he needs a diaper change and give it some air/sunlight for a bit. If it turns into diaper rash than really cake on the vaseline/a&d ointment to protect his skin which creates a barrier from more irritation as the skin underneath tries to heal.
Just keep giving lots of love and hopefully it'll be over sooner than later! Blessings!!