H.D.
It all depends on what his divorce/custody papers say, unfortunately. If they were married for more than 10 years (this depends on the state too) then she will get alimony for a certain amount of time. Child Support is based on guildlines according to his pay. If his salary goes up or down OR if he has another child by his new wife he can petition the courts to have it adjusted. Usually if he has medical/dental insurance then the children will fall under his, unless she has it as well, then it should be worked out between the two of them (or the judge will decide). Co-pays and expenses not covered by insurance are usually paid by the person that has custody of the child or split 50/50 unless otherwise stated. Unless it is stated in the custody decree he should NOT have to pay for all the other extras unless he wants to! Things change from state to state, county to county so he needs to find out exactly where he stands. AND she has NO RIGHT to withhold the children if he doesn't pay his support for whatever reason.
He should see a lawyer and find out exactly what his rights are. Oy, I know more about this subject than I ever wanted to, trust me. =/