What relation is my grandson to my mother's sister?

Your grandson's half-brother would be the son of your son's wife and another man. This means the child has no blood relation to your son and, thus, to you.

If the child was not adopted by your son, then he would be your son's stepchild. By extension, you could call the child your step-grandchild, but this isn't really a commonly used Word.

If your son has officially adopted the child, he would be his adoptive son and, by extension, your adoptive grandson.