You need to get a fence tester, sounds like your fence isn't hot enough. It needs to be on at all time, and perhaps another strand of tape. It needs to be HOT enough to land you on your ***. It probably has a ground somewhere so it is a mild shock.
Agreed - something is wrong with your fence, especially if (as you seem to suggest) he's standing there with it laying across his belly half over it. A properly working fence will yield a much different response.
Is it a solar based unit , or plug in? If it's solar chances are the battery isn't getting enough charge to maintain fence voltage - either the battery could be bad (how old is it?) or the solar panel isn't aimed correctly or isn't receiving adequate sun to charge the battery.
If it's a plug in unit, the only reasonable explanation is that it's grounding out somewhere and the fences voltage is depleting to ground instead of maintaining the charge in the wire itself. If it's a unit with a voltage/strength control check that it's cranked to the top, as well.
Grounding issues can also effect solar based units as well so don't rule that out no matter what.
I agree you need a tester though. I've been known to do the "finger test" before, and trust me, if a fence is working properly you'll know it as it'll just about knock you on your butt when you get hit.
