The blood test is notoriously poor. You said a couple of weeks since you last had sex with someone else. If you contracted HSV 2 but had not had enough time to have made antibody, it could be that you infected your current partner. You should do another IgG test in about 10 weeks. You could get a swab test kit and swab yourself if anything shows up in either of you. I think it would be extremely unlikely that you would now get HSV 1 genitally and he goes from negative to positive on the HSV 2 antibody test.