I needed one a while back and bought a Porter Cable 16ga model; I already had a compressor. It is one great tool! Well designed, well balanced, plenty of power, easy to use. You can get one in a kit with a small compressor for about $300; you may even find it on sale nowadays. That compressor is plenty big enough.
If he would need it more for small-scale work, an 18ga model may be enough. But I think that size is too light for molding work.