This thread is proving to be interesting.
This year we Brits have a problem - too much rain & too much good grass. Laminitis looms all about us. We have to watch for overweight.
Then there is the other problem that a pasture reserved for horses alone is not a balanced natural feed - we have to give supplements.
I am even feeding local, repeat local, honey in the hope it will help with a suspected allergy problem.
Personally with Delta I watch her routine - ie how she gets to eat. Obviously with a horse a little feed constantly throughout the day and even night is preferable to all of her feed in one or two feeds. I noticed Delta will graze constantly - all day - but the grass is very short so each mouthful is not much feed. So she comes in during the day. I put her out an hour before it gets dark. SHe gets a handful of pasture mix at mid day in the stable
The ad lib feeding from a round bale seems a good idea - I'd not thought of that before. But our big baled hay is subject to mould - so I am not sure.
My horse also gets apples, carrots, parsnips and swede - together with all the trimmings from household vegetables.
The field she is in has a lot of natural weeds including various clovers. I notice that when I give her a certain strip - sometimes she is real keen to get back to the same spot the following day. Obviously she likes certain weeds more than others.
But one thing I would say - and I am not a TB person, so I am no expert on the matter - your Puck doesn't look any ribbier than some of the TBs hereabouts.
Of course you have not mentioned the work routine - does your chap need more feed to offset his work load?
I use the weather as a slimming aid, If its cold I leave her out just a bit longer. If its raining real hard, then I let her get wet. She is never left out in the sun for too long (even when it shines) - she gets sunburnt.
For you of course, everything I have said is in reverse. Perhaps we could do an exchange visit. I guarantee we could fatten your chap up. Maybe you could slim down my Irish Huzzy.
Best of luck
Getting this weight problem right is one of the biggest problems to solve for the private owner. But my girl is 1/2 Irish Draught and 1/2 Connemara and she was bred in one of the wettest parts of Ireland. SHe is what's known as a good doer. And she never stops eating - just in case the famine might come back. If I ever have to choose another horse, then I am going to think more about matching the horse to the climate.