For £100, a s/h Gamo Compact, or for less, a new Gamo P45 (basically the Compact, a slightly less sophisticated trigger and without the wood grips, which may be too big for a lad).
The Baikal 53M , a springer, is also robust, well made and accurate, though the accuracy depends very much on the mutt on the butt! If he has patience, a 53 will really teach him to shoot pistol.
Walther CP-2 Match, FAS 604 & Tau 7 target pistols, Smith & Wesson 6" & 4" co2 pistol, Crosman 1377,
Baikal IZH 53 pistol, Gamo CFX Royal,177, Umarex SA-10 CO2 pistol.