N.B.
I think the best way to handle this is to ignore it, and I mean really ignore it----like turn your back and ignore it. Take your baby and the girl, and sit down to read a book or something, and exclude him. Sooner or later, he'll want to be a part ot what you're doing. Ask him if he can be a good boy, then include him. The second he fusses at anyone, put him on the floor and continue what you were doing like he's not there. He'll throw a fit. Let him---ignore it. when he settles down, ask him again if he's ready to be a good boy, and include him enthusiastically. They really are all too young to expect them to play together without a problem. Just be calm and firm. At least they're close enough in ages to nap at the same time! Don't forget the "Me" time every chance you get!