Sure it'll work. Not ideal, but a lot of ladies have shot deer with low poundage bows. Heck, look at all the deer killed with recurves in the old days when speeds were way below 200 fps. If he practices he'll beable to pull a few more pounds in no time. At this point keep it low, work on form and the muscles will build. Work out a plan after a month or so to crank it up a pound every other week or something like that. Penetration will be no problem at 20 yards if you have a good SHARP fixed broadhead, maybe a 2 blade with some bleeders. Bow tuned to the optimum for himfor the best arrow flight will also be a must. If form is wrong or irratic, penetration will suffer.