My brother has a son that will soon be 18 years old. Is he still obligated by law to continue child support payments to the mother if his son decides to go to college? And if so, is he obligated under California law to continue payments until he graduates from college?

Most states do allow the collection of support for college attendee's.

It may or may not be indicated within the divorce decree as to what age this extended support terminates. Some are 23, some are 25, if still attending college.

I would refer to your brothers divorce decree for the details.

I have read, in some circumstances, such as the child is no longer under the support of the custodial parent, working themselves, paying their own expenses, paying for their own college, and this evidence being presented to a judge has resulted in lowering the support, or in some cases, ending it completely.

It will be up to the divorce agreement.