You don't need to spend a lot on a red dot, but if you want to there are a lot of really nice options in the £150 and under bracket.
There are many options. ACOG style, scope style, reticule style.
ACOG's are a closed sight, 1x, 1.5x, 2x, 4x magnification. With an illuminated dot (red, green, blue). 1x and 1.5x magnification is better for airsoft as you don't need the higher magnifications. ACOG's are heavily cloned and you shouldn't need to spend more than £50 for one with the options you want.
Reticule style is an open style, It's nothing more than a bit of glass with an illuminated dot projected onto it. Again heavily cloned and you shouldn't need to spend more than £50 to get the spec you want.
Scopes are different. Scopes generally need a special ring mounting system, 25 and 30mm rings are common and they fit into the 20mm rails. A scope is very much a user thing, and the budget end of the market are generally fixed 4x magnification with large illuminated dots. The budget rifle spec (£80+) is where things get interesting with them. The market is very competitive worldwide, And for your budget you can get a very good rifle spec scope, with variable zoom, illuminated dot, weather sealing, designed for use with gloves. It's in this market your budget falls, but you should be looking at live fire webshops rather than airsoft ones.
Models like the Nikko Stirling 30mm Diamond 1-4x24 IR, fall right into this bracket. It's a rifle scope that can be used like an ACOG, but gives the option to dial in a magnification level that suits the situation. It also has a far finer illuminated dot that any airsoft marketed product, and has sight marking for when the batteries die and you don't have any on you. It's also got a focus ring. Uttings are selling it for about £130 atm. You will also need the mounting rings. And there are other makes as well that do the same thing, like I said competitive market.
It should be pointed out however that a scope or expensive red dot is going to get shot at, and it's totally unnecessary for airsoft since our guns are wildly inaccurate.
http://www.uttings.co.uk/p122015-nikko-stirling-30mm-diamond-1-4x24-ir-rifle-scope-ndsi1424/#.WptYQHxpHZI
http://www.nikkostirling.com/content/Products/Diamond/Diamond Hunting/NDSI1424.html
If you want to treat your son, you can always take him out to the nearest live fire shop (most citys have one) and let him pick within budget. Plus you get to look at all the live fire gear and have a lads day out!