First of all, dump the replacer and feed plain old whole cow milk from the store. While some kids will do fine on replacer, others cannot tolerate it and that includes the expensive replacers made especially for kids. They may scour or worse, suddenly blow up and die. Trying to get mother raised...
You may have to restrain her so he can nurse I once had a crazy Nubian that pitched a royal fit at being milked. She would kick, jump up and down and throw herself on the ground. She didn't want her kids to nurse. She didn't want to be milked by hand or machine either. I went out to the barn...
You can set up a heat lamp. You can also make a cubby he can hide in and that will help him retain heat. A dog crate , a wooden box, anything that is small and that he can get into.