You really just read your dog. That is the wonderful part of being an amateur with one (or a few) dogs.
We could not do water force with Rocket Dog because it was winter. She was older than "normal" when we seriously got to water. But so what? I have come to believe that she is on her own time line and we are not going to be setting records for the youngest dog ever to go to the Master National.
You will have warm water sooner and longer than those of us in other parts of the country. You don't have some narrow window to force your boy in to. My sense is that if he's worked hard and made good progress with FF... then give him marks. And even when you start yard work with single and double T... you can certainly give him field work. Just don't run blinds or try to handle him (to any marks) until he's ready for that.
I'm not a hunter. So I wasn't being all anal about getting her ready to hunt as a youngster in her first (or even second) hunting season. The pro's I know don't want client dogs to be hunted until they're really solid. It's easy to see why when you go to a HT in the Spring. The dogs that were hunted months earlier are just crazy. The dogs that were not, remembered and retained what they'd been taught the previous year.