11 Yr Old Neutered Male Cat Acting Oddly?

Could be I've not noticed this "new" behaviour before and I recently had to put down my 8 yr old female cat but....my healthy 11 yr old male contracts and shakes his hind legs while lying close to my other cat and sometimes against me. He has a semi to fully exposed while doing this. The other male gets mad and swats because of the hind legs repeatedly pressing against his stomach. He doesn't have any discomfort or reluctance using the litter box. Do you think it is just sensitivity and nothing to worry about? He's got a solid build and has always enjoyed a little rough housing with a pseudo "". He is VERY gentle natured and loves to groom and nurtures other cats. Thanks!