Have you throughly checked in his paw pads, like between them, ticks love to get in there, pebbles and its spring so maybe a sticky burr or something from the lawn is caught.
Last week Laci would NOT walk and kept holding up her paw, I didnt see anything until I came home and I found one of those sticky burr things between her pads , it was like little needles and hurt me to pull it out. It could even be a tiny pebble or stick.. you really have to look closely as their feet are so tiny and even the littlest pebble can be uncomfortable.
Ticks also like to hard there and its hard to see/and find them
