My oldest cat, Augustus, is prone to crystals and he has done the same thing a couple of times. He will leave little droplets of urine on the floor and strain in the litterbox for quite a while. He also walks around and squats like he is going to urinate - but only a drop comes out. It could be a blockage from crystals, I would take him to the vet ASAP. I adopted Auggy from a local vet - he was surrendered because of his crystals. He was almost dead when he came in because his previous owners ingnored the fact that he had been sleeping in his litterbox for a few days. He had a blockage and luckily the vet saved his life.