I had one at the Dell too. And have also been to at least one at St Mary's (when I was about 11). For kids, especially those who are Saints fans, it's great! A chance to look round the stadium and even have a jog around the pitch, so I think your son and his mates would enjoy it. Can't say I was really old enough to be thinking about how good value for money it was though!
One thing that did dissapoint me is that at the Dell, they used to have an indoor football room where you actually played a few 5/6 aside matches, where as at St Mary's it was just kicking a ball at a target thing. Still fun when you're young though!