A 1995 bike will probably onto take a very short travel fork; 60-80mm would probably have been normal in the mid-90s. That will limit your choice somewhat if buying new.
Is it a 1.1/8" steerer? If not, then you'll really struggle to find anything that will fit.
Forks should (and most do) have a quoted 'axle-crown length'. This, not surprisingly, is the distance between the fork's crown race and the centre line of the axle when the fork is uncompressed. Ideally, you are looking for a suspension fork that is slightly longer than this dimension on your rigid forks, so that the length is similar when the fork sags under the rider's weight. Having said that, a mid 1995 bike is likely to have a steep head angle compared to current models (71 degrees versus 68-69, say), so while a longer fork will make the steering 'slower', it may not be disasterous - especially if combined with a shorter stem.
Since your lad is light, an air-sprung fork might be a good idea as these are more adjustable than coil forks. Softer springs are available for most coil-sprung forks, but tend to cost ~£30, even for budget models.
Recommendations: Currently, the budget Rock Shox forks are good value.
These are probably amongst the best you'll find for that vintage of frame, but might be a bit over-sprung for a 5 stone rider.
You might also consider second hand. If I was doing the same, I'd be inclined to watch Ebay for some late '90s Marzocchis. I have some old Z5s on MrsO's bike which have done good service over 10 years or so (5 or 6 on my bikes, working hard). If the stantions are OK then these can be made like new with replacement bushings and seals - and the seals are good so the bushes rarely need doing. They're also simple (little to go wrong) and air-sprung. Unfortunately, they don't come up very often...
