Welcome to the forum.

As above, having a little more information would certainly be useful, like how big is your son and what you term as "budget". Most on a "budget" would favour a light to middleweight springer and there are a fair few candidates available. Looking at your location, if you're able to travel you shouldn't be too far away from some well stocked shops offering an extensive choice, like maybe Solware at Tamworth, DAI (Direct Arms Imports) and City Air Weapons. Nice to get a feel for size, weight, balance etc.

Feel free to shout up for further advice and assistance. The BBS collective are a friendly, helpful bunch.