With MS, nerve signals can get jumbled up. That can cause strange sensations, such as a leg feeling cold or hot. These are called paresthesias.
Is your husband experiencing any other symptoms? Something new like this could signal a relapse. It is worth calling the neurologist to see if he/she wants to see your husband. If a relapse is beginning, the neuro might want to prescribe steroids to cut it short.
In general, however, paresthesias are harmless but annoying. If numbness is involved, you do need to be careful not to burn yourself, cut yourself, or otherwise injury yourself when you can't feel it.
Please let us know how he's doing.