This got me thinking... Maybe Jess' stress is due to Milo's 'un-neuteredness'. Not that he's stressed by Milo attempting to mount him but more the smell which he gives off which as humans we can't smell but cats can and are very sensitive too and find threatening. Did the start of his stress problems coincide with Milo being old enough to give off 'the smell'?
Jess will be pleased that Milo gets to go to see the needle man instead of himHave you thought about getting him chipped whilst hes under?
Good job too really last night he was trying to "get it on" with Jess, who looked mortified!