The best way to get your breastmilk going again is by having him suckle. It is the most efficient way for your body to release the prolactin to signal milk production. A breast pump can do that to some degree, but not to the extent of actual suckling.
I'm not sure if breastmilk will completely resolve his GI issues. If it is a malfunction of some part of the GI system, it may not be related to what he's consuming, but it can't hurt to try breast milk.
It's been a long time since you nursed, so I hope your supply isn't compromised enough to give it a go again.
Good luck. I hope his issues are resolved soon.