Hi
It sounds like he might have an old dislocation of his hip or even an old fracture there. You can't give any human painkillers to cats they can be toxic, and in the UK where I practice there are regulations that prevent me just handing out medication to pets without seeing them. If he lets you touch and probe his leg are you sure you cant try to get the cat into a carrier to be seen and diagnosed. Then you could get something safely prescribed for him. That would be best.
Cathy Wickenden BVet Med MRCVS
Barton Lodge Veterinary Centre
1 Midland Road, Hemel Hempstead, Hertfordshire, HP2 5BH
Tel: 01442 216048 24 hours
[
www.bartonlodgevets.co.uk]