Looking for a good small subwoofer...

He could get a pair of SVS SB1000 Pro subs for only $1350.  They’re basically 13” cubes with 15” depth, are sealed with a 12” driver, will get down to an honest 20Hz (-3dB), and includes integration software that lets you dial in the subs from the listening chair with your phone.  Also, they offer a completely risk-free, 45-day trial period including shipping both ways so if they don’t work out for some reason he can just send’em back no problem. 
https://www.svsound.com/products/sb-1000-pro-subwoofer

Best of luck. 

Steve:

I would strongly suggest a Totem Kin powered sub. They have them on sale right now at the Totem website for $500 ( regular price is $800). Sound and build quality are off the charts. Probably the smallest sub I have ever seen too. Sealed enclosure, 8" rigid carbon fiber woofer, magnetic grills. One of the most un-colored subs I have owned. Does not muddy up the lower midrange like most subs priced below $1000, even when the crossover is set above 100hz. This sub actually improves the lower midrange when used with small to mid size main speakers. A true audiophile subwoofer for those who appreciate music and not "boom"!

Very happy with Rythmik subs.