If your son gets an offer at Hampton at 10+ he'll be invited back to sit the scholarship papers the following year. You will have to pay the deposit for a place, but if you don't mind using that as insurance, and giving it up if he gets a place somewhere you decide you prefer, there's nothing to stop him sitting as many 11+ exams as you like the following year. You might, for example, be thinking of trying Tiffin or one of the other grammars and therefore consider spending £1,000 (I've forgotten how much it is) to hold the place, but let it go if he gets a Tiffin offer.
If he doesn't get an offer, it won't be held against him in the 11+, so in that sense you have nothing to lose, unless you think he'll be very upset. There's also no interview at Hampton at 10+ and the exam's on a Saturday, so they don't even have to miss school.
My younger son sat 10+ deferred this past year at both Hampton and Colet Court, and was offered places at both. We pitched it as a chance to have a go and a practice for the 11+, but our attitude was that he would probably not get in (as we genuinely didn't know). His older brother had done 11+ so this was our first experience of 10+.
It was a lot less stressful than the 11+, as relatively few children are sitting the exams, and also both parents and children know they've got another chance next year. Year 6 is now extremely relaxed for DS2 (no tutoring/past exam papers), no worries about where he's going etc. So unless your son is very sensitive or easily crushed by disappointment, I'd recommend it.