Unless he also shook hands with, say, your female friend that day, you'll never know whether he usually shakes hands like that with everyone or was softening his handshake for whatever reason (as others have said, he could lighten up for those he sees as women or for those men he sees as smaller or less physical than he).
You can always, as others have said, put more challenge into your own handshake. But I think that what's bothering you is his possibly reading you as female in the first place. So I think you should just give your transition a chance to further masculinize you and take the "you must be a girl" handshake off the table. As someone else has already noted, nine months is nothing.
There are strong handshakes and strong handshakes. A lot of men I've encountered seem to have no idea that their handshakes are over the top. Others use a gorilla grip to gauge or dominate other men, and I think it's sometimes evidence of insecurity, not confidence. For example, when I was pre-transition, my ex and I met his sister's boyfriend, a big and imposing retired fighter pilot, for the first time. The boyfriend put my ex-partner's hand in a viselike grip; my ex just let his hand go completely limp to show that he was aware of the game and wasn't having any of it. This response seemed to throw the guy. Before that, the guy had shaken hands with me quite amicably without crushing my hand; as I said, I was presenting as female. But later on, when he found out that I was smart and articulate and not shy about being that way, he started to become wary of me and seemed to see me as a possible intellectual threat. I'm sure that my ex's intelligence was threatening as well, but I was a "woman." The guy avoided both of us after that encounter. All three of us--my ex, his sister, and I--were laughing about the whole affair later. I think that under all that size and bluster, the guy was just insecure as hell.
