Liz,
Even if you decide to go for a lump sum in terms of a property/alimony settlement, in all likelihood your state would not permit a lump sum to be paid for child support. Child support is the right of the child, not you (as the custodial parent), and if he makes more money in the future, or if inflation dramatically decreases the value of the dollar, or if other circumstances should occur (for example, god forbid, that your son is injured or ill and requires very expensive medical treatment), that lump sum might not account for those eventualities.
Talk to your lawyer about it--you might not be allowed to do this even if you would like to.