Most likely we are either dealing with a partial lactose intolerance or a secondary lactose intolerance .
Since this is happening first time in 30s it is suggestive that his GI bacterial flora might have changed post a viral or bacterial infection or a course of antibiotics.
You can try NOT to drink milk on an EMPTY stomach and see if symptoms lessen, otherwise use Lactaid milk.
Don't know if lactose breath test is available but a two week lactose free diet is a good test in itself and not drinking milk on empty stomach will help. Taking non-lactose containing probiotics to readjust the bacterial flora may help too.