My husband is un the military and he is paying child support to his ex which is a lot of money for only one child. He agreed on payinf that amount but now we have a baby and he only wants to pay what is enough to take care of he other child. Can that be posible?

He will need to file a modification of child support for the other child to lower his monthly support. This will all depend upon his income. Has it changed and gotten less, stayed the same, gotten better? Just because you have another child may not be enough of a reason for the modification to have a judge agree and lower his obligation. Also, has her income changed in any way? This also could be used as a good enough reason for a modification of the child support amount as well.

Ok, he should not have just agreed to any sum of money. IS there a legal child support order in place though the court or is their a unofficial agreement ?

Assuming it is a court ordered amouint, he will hire an attorney and file a motion to have the amount admended. Now most states have a system that takes into consideration, many factors, some they will take your family income and the other parents family income, some just income of the two parents, and in some states just the income of the person paying.
So he can get it set according to your state rules.