We've sent tups off straight after tupping and had 'em minced just in case - but the meat was beautiful.
If you want to keep him through to spring, he'll be fine anytime. The meat tastes sweeter once they've been eating grass for a few weeks. Where I am, the grass thinks it's still winter until halfway through April, but we're often short of grass for lambing, so the tup would either have to hang around until mid May or so, or be going off without much new grass in his belly. Personally, I'd be guided by conditions and management considerations, rather than optimising meat quality for one tup. But if you have no constraints, then whenever he can have been having nice fresh grass for a month.