I've always been told that when at cat stops using the litterbox and nothing litterbox-wise had changed, they may have a urinary tract infection or something. You should keep an eye on him, and if he keeps doing it, maybe a visit to the vet would be necessary.
First of all, have him checked. It sounds like he cannot sit down properly in his box. Also it might possibly be a 'senior' thing. They sell 'senior' boxes these days, they are basically lower than usual boxes and easier on the old bones to step into. Only the vet can tell you though, how old your kitty really feels (painful joints maybe etc).
The other thing that comes to mind is not liking the litter type. Have you recently changed it? Maybe it hurts his paws?
You might also try adding another litter box...one for peeing and one for pooing. Dulcinea (our VERY emotional calico cat) use to do this...but after we provided two...the behavior stopped and now she uses one for pooing and the other for peeing and never misses or does either job outside the boxes.