If he's used to fairly light weight, power, wanting a gas saw and good a/v etc, then new: MS241cm, Husky 545 or 550xp (or the J-red versions), Dolmar 421 or a Echo CS490. Varying price points, but all are good tools and will be easy to start and live with. Used - Husky 346xp, 350 or 353, Stihl 024 or 026 Pro, Shindaiwa made a good 50cc saw and John Deere (via Efco) made a nice 52cc saw that's a kick to run.
I have 4 or 5 brands of saws in the barn and have different favorites in the different sizes. For 40-50cc saws I really like Huskies, but I am VERY impressed with the MS241CM Stihl, especially ported but you're talking some $$$ at that point. Unless it's a surprise, take Him shopping and let Him try some different things and see what works for Him. Doubtless, like the rest of us, he'll know when he's found it whatever "it" happens to be.