It is certainly not illegal in terms of against the law of this country as i have one and some sites allow up to 500 fps for snipers. Looking at the ambush adventures website i gather that it is mainly cqb due to a relatively low 328 fps limit on .20g bbs. They were probably saying that it would not be allowed for use at their site.
If your son really does want to start airsoft at a managed site, then it is generally said that starting with a sniper is not a great idea as to be competitave over others, they often require more money to be spent on upgrades as well as possibly being boring. However, if your son is happy with staying with his friends in a field it should be fine, but you may want to consider an aeg like a combat machine or similar or alternatively read this - http://www.airsoft-forums.co.uk/index.php/topic/17294-ians-guide-to-noobism-on-a-tight-budget/
Hope i helped