Wagtail
Horse servant
But a horse with a condition score of 2 is not thin - it is described as "moderate" by WHW condition score - that is unrelated to whether he is a welfare case. To be described as thin, again by condition score standards, he would have to score at 1 - which he is nowhere near.
OP, he is not thin, he could use some extra hay, but does not need, nor will he benefit from, you ceasing to work him within reason.
Actually, rereading the WHW guidelines, I would put him under a two as his spine is not well covered. He is thinner than most racehorses you see in terms of his spine and ribs. He is not meant to be built like a racehorse and as tucked up and thin as they are as he's a big boned WB.