I'm guessing that the disc rotors on your son's bike are 160mm.

The following adapter might work but I wouldn't guarantee it: http://cgi.ebay.co.uk/KCNC-Disc-Brak...item415955e79f

Another option is to replace the front brake to one that will bolt straight onto the fork like the following: http://cgi.ebay.co.uk/Shimano-BRM-49...item43a633432f

Here's a bit of talk about that brake: http://forums.mtbr.com/showthread.php?t=168386

There are better mechanical discs but it sounds like you're on a budget and as the bike isn't what you call top quality it probably isn't a good idea to spend too much on it.

Some other brakes you might want to look at: http://www.chainreactioncycles.com/C...0&SortBy=Price