I think that a lot if it is just needing more mental stimulation and attention. I wouldn't necessarily say that he's missing the work persay, more just missing the interaction, and the "job" that he had before. My dad's horse was at least 35 years old when we had to put him down, and we never retired him. We put him in semi retirement when we got him, he was a national champion barrel racer in his younger days, but we just took him out on trail every weekend, and I would work him during the week. Up until the day he colicked, I was still jumping him over really really small stuff, and barrel racing him, granted a lot slower as he got older, but he was still doing a lot of stuff. On the couple of occasions that we had to lay him up for a week or so because he got a bit sore, he definitely got upset, and wanted to come out and do something even if it was just handwalking around the barn. Some horses do great doing less stuff, or being retired, while others absolutely hate it, and need to be taken out and worked in some way to stay healthy and happy. I would recommend going out even if its just for an hour or so a day at least 3 times a week, and do some stuff with him, like playing with the ball, just to stimulate his mind and body, and give him something constructive to do. BTW he's adorable.