Not all horses handle change well. It could be that he is depressed and missing the security of his previous routine. Different hay, different water, different turn-out arrangement, different view, diffdiffdiff. From a psychological pov, his security has been rocked, his place in the herd disrupted and the daily routine altered. Not in a bad way, just different. Geldings, I find, generally tend to need more reassurance and reaffirmation about their place and purpose.
Even if the same horses and people are still involved, the view changed, and that can mean learning a whole new set of environmental influences that will dictate the need and degree, or not, of survival and coping mechanisms. This might sound a bit odd but it is possible that he is exhausted from coping (exhaustion can open the door for depression). Spend as much time with him as possible, if there is any chance for the opportunity to replicate any of his previous routine, that might help. Change can be difficult, and not just for animals : )
Also, never wrong to consider health issues, some afflictions can be very subtle and horses, by the time WE notice something NQR, can have been harbouring ill health for a while, not wanting to be seen as vulnerable.
I wish you an easy solution that brings your boy around soon : )
Sure there's right and wrong but mostly there's just a whole lotta different.