Agree with Lucky...definately not a reaction to the other cat!!
You need to take him to the vet, the cat is obviously in discomfort and this could be caused by anything, stomach pain, back pain, sore legs....... I could go on but I won't
A one year old could get arthritis but I really don't think that's the problem as arthritis in young animals while it does happen, it is not that common either. Impossible to help unless you take him to the vet for a look-see.
Let us know how you get on.