It took my husband years to be ok with the IDEA of me potentially being with another man (which has never happened yet so when (if?) it does it will be interesting to see what the reaction to it is then). The thing is that there is an awful lot of societal conditioning that is ingrained into men about having to "protect" their women from other men. It's not something that you just get over overnight. You've only been together a couple of months and this is LAMs first exposure to poly. Give him some time for the ideas to sink in.
Everything will be ok in the end. If it's not ok it's not the end.