There is only one healthy way, help him find a new girl. I say healthy because time heals all but who knows where his mental state might be.
Even if this girl is only an actual friend not a partner. Unfortunately, you cannot fill this role, so please do not try.
It is actually not common for first romances to last unless it happens mutually during puberty. Then there is actual chemistry that comes into play.
However, he is probably feeling inadequate at this point and needs his self esteem lifted. (Low self esteem is a girl negative.) So invite your friends and their daughters over for cookouts as often as you can until he gets into another relationship.
Do not try to force him or a girl. If he thinks you are doing this he will move in the opposite direction.