I'd put mom in the roundpen, to dry up, leave colt with the " herd", then start, after mom is dry, feeding her extra to gain back her weight.
I let my mares wean the babies, usually happened when they approached a year, but it was a herd situation and very easy on all involved. And I had moms and babies on a feed, so babies didn't " overuse" the moms, so I didn't have weight problems.
What I would do in your situation is take her out and in the roundpen starting with an hour, and watch how colt holds up without her.
If the gelding is boss, you shouldn't have to be in such a hurry to geld the colt. He won't stand much chance. But I would get it done before spring for sure. MHO, I don't believe in gelding super early.