What can I do for my son he has a lot of pain on his heel? We were told it was severs disease. 

See below. If pain is that severe then he needs to be either casted or put into a cam walking boot to keep pressure off the heal, once the pain subsides and orthotic will help to control the foot motion and control the pain.
Decrease sports . Activity. Wear well cushioned heel cushions and if any degree of pronation or foot flattening while walking then an orthotic is preferred.
Conservative care. If a diagnosis of sever's disease (or calcaneal apophysitis) has been made is essentially an irritation of the growth plate of the heel. Good heel cord stretches (i.e. Runner's stretch), heel cups, rest and ice are excellent measures. The good news is that when the growth plate closes, this will resolve.
Some things may help. For severs disease (inflammation at the heel bone near achilles' tendon), one can take Motrin 3 times a day for a few days to decrease inflammation. Decrease sports activities, and use a cold pack on the heel 3 times a day (after activities). Use a good arch support + shock-absorbing heel cushion, to prevent pronation from flat feet. Do calf stretches. If not better, see orthopedist or podiatrist.