Sounds like the collar is not wide enough to go over his head. Usually collars do not need forcing, to fit past the eyes on down the neck.
Collars do come in widths, maybe shaped with a wider top or bottom, than the other end of the center hole.
Personally, I would take the pony to the shop, have them fit him there. You can try on a number of collars, see what fits best.
There are collars with a latch that allows a total opening, but then you have to put the hames on the collar and tighten them down on the animal. Seems like that is how my Grampa hitched up his draft ponies. Maybe that is easier fastening the hames with draft harness than it is on carriage harness. Our hames get left on the collars all the time, but traces go on the tug buckles. Traces are not permanently on the hames eye like draft harness.
If you can’t take pony to the collar shop, you can get measurements with using carpenter squares. You put two squares with the long ends together for one side, short length on each end of the 3 sided box you now have. You can fit the short ends on top and bottom of neck for length, with the long ends together adjusting for size. [ You can then take the two short ends on each side of neck, sliding the long ends together until you have neck width. You should look at his neck shape too, is it oval, same width top to bottom? Maybe narrow at his mane. Maybe really cresty, narrow at the windpipe. Neck shape can make fitting difficult in the “one-size-fits-all” type collar. A really wide headed horse with bulging eyes, can be wider than the collar he needs, if you want a closed top collar, but no problem with the open top collar.
There is really a lot to fitting a collar and keeping your horse happy in his work. I would strongly advise taking pony to the shop for fitting, they are experienced. But at the same time, don’t just “settle” for one off the shelf, if you see collar has obvious fit problems they can’t explain to suit you. He will probably need to get his head down to pull a load, so he will need a little room on the bottom, but not enough to let collar rock on his shoulders. We always use false martingales on collars, keeps the bottom collar end down, not moving around in work. You might look at Lost Farmer’s videos of his ponies, they are dressed for work in collars and draft harness. Could help develop your eye for “correct fit” looking at other animals.