Does he not do well on hay alone? I have a mare that was retired at 10, she is coming 12 now. Currently bred for a April/May foal. However before being bred she was on just hay, and pasture during the spring/summer/fall. Winter we just upped the hay and added in some BP as needed. She's not a hard keeper, but is a large mare. Of course, not all horses do well on just hay so that's why I ask if that's an option for your guy or not.
Beverley - 17.3h moose mare, retired to broodmare, Jolie - 2018 Appendix filly, Maybelline - 14.3h APHA mare