Our 18 month old boxer has been a bit off his food for a couple of months now and has been loosing weight. :( He will take human food but has little interest in his own food. (even if offered nothing else, after a couple of days I always give in and give him a bit of toast!) 
Some days he will eat the lot but usually just picks. The vet reckons that it is his age and he has got too much on his mind to eat (ie sex). He doesn't show any signs of this however, he has never tried to mount another dog (or person) and doesn't seem to be marking any more than usual.
He only has one testicle and the vet is relucant to castrate him unless he has to. We are to take him back in the New Year to see if his weight has increased and meantime do all we can to make him eat.
Has anyone had any experience of this? Or castration with only one testicle?
Thanks
p.s when I say loosing weight he is now 30kg down from 33kg so hardly fading away but still worrying when he doesn't eat.