My dog :(???? what should i do?

ok well he hurt his leg and we took him to the vet and he parcitally tore his acl in his back leg hes getting surgery but not for a few months and hes getting old hes 8 and hes only going to live till hes nine but we want his last years to be happy and less painfull so were going o with the surgery but he only walks on 3 legs and has a har dtime wants some medicine we can give him to hold him off till his surgery