If you need to clip the horse for overheating reasons then I'd say sure go ahead.
It really is not going to do anything about how his summer coat comes in though, aside from if you clip too close too late you will take off the growing-in tips of the summer coat hairs which makes the summer coat look duller.
If it were me I would probably be inclined to just keep after him with a rubber curry and grooming mitt and keep him as shed-out as his body allows (i.e. whatever hair IS loose, get it offa there) and see if you can avoid clipping. Even if he has some longer patches of hair left in late May, which I would not say is terribly unusual, they are likely not going to be *so* much as to make him overheat. Yeah, there may be a day here or there that is unusually hot for the season and you may wanna take it easy with him; but that is a heck of a lot easier than clipping
and IMO a bit better for the horse.
If you are CERTAIN that you MUST clip large areas to avoid overheating even at minimal-normal levels of work, then your two best options would be either a) do it real soon and then keeping a turnout sheet on him til the weather warms (or even a light blanket at first), or b) wait til as late in the season as possible so that he *looks* body-clipped. (As opposed to 'c' - doing it halfway thru the summer coat growing in, which tends to result in not looking as nice. If that mattesr to you)
If it were me I would also be inclined to have the bloodwork re-run to try again to make sure there is not an underlying issue causing this that should be addressed.
Good luck, have fun,
Pat
It really is not going to do anything about how his summer coat comes in though, aside from if you clip too close too late you will take off the growing-in tips of the summer coat hairs which makes the summer coat look duller.
If it were me I would probably be inclined to just keep after him with a rubber curry and grooming mitt and keep him as shed-out as his body allows (i.e. whatever hair IS loose, get it offa there) and see if you can avoid clipping. Even if he has some longer patches of hair left in late May, which I would not say is terribly unusual, they are likely not going to be *so* much as to make him overheat. Yeah, there may be a day here or there that is unusually hot for the season and you may wanna take it easy with him; but that is a heck of a lot easier than clipping

If you are CERTAIN that you MUST clip large areas to avoid overheating even at minimal-normal levels of work, then your two best options would be either a) do it real soon and then keeping a turnout sheet on him til the weather warms (or even a light blanket at first), or b) wait til as late in the season as possible so that he *looks* body-clipped. (As opposed to 'c' - doing it halfway thru the summer coat growing in, which tends to result in not looking as nice. If that mattesr to you)
If it were me I would also be inclined to have the bloodwork re-run to try again to make sure there is not an underlying issue causing this that should be addressed.
Good luck, have fun,
Pat