While I didn't get a rash, Lamictal was bad for me. I ended up with achey, flu-like symptoms and swollen lymph glands. I tried it twice, almost a year apart and had the same reaction.

As far as it passing through breast milk, it probably does, and if your son has a rash, he may be allergic to it. I would call your pdoc to let him/her know what happened, and stay in touch with you son's pediatrician.