Oh, lawd, absolutely everything. Teaching him cute little magic tricks, reading books to him, helping him with his homework, all those things. He would have been happy just to do that in general but knowing that he's going to be partially responsible for raising Jack and helping to shape his values is both terrifying and so exciting for him.
As for helping Jack cope without his mother, I think one thing he might do is what Reid does (or used to do) in canon. With his mom away in the hospital, he would write a letter to her every day. He did that because he found it difficult to speak to her over the phone or go visit her since he felt so guilty for putting her in a home in the first place, but it'd be different with Jack. He'd probably suggest they both write letters to their moms and explain it as a way to catch their moms up on their lives if they ever do show up in Darrow.
no subject
As for helping Jack cope without his mother, I think one thing he might do is what Reid does (or used to do) in canon. With his mom away in the hospital, he would write a letter to her every day. He did that because he found it difficult to speak to her over the phone or go visit her since he felt so guilty for putting her in a home in the first place, but it'd be different with Jack. He'd probably suggest they both write letters to their moms and explain it as a way to catch their moms up on their lives if they ever do show up in Darrow.