Not sure what model Hi-Boy that your son has, but wife & I have matched pair of Hi-boy Pro's that we use only while RV'ing.
The pro series has a few more features, adjustable rear suspension, very light, more power, etc. Good range but they wont win any speed records though..23-24mph down hill is about max. They are great for hauling stuff (laundry, firewood, mail, snacks from the C-store..lol)
I also have a stripped down over-volted Gt-Sport, I got simply to hot rod. Factory will go 30'ish mph flat out. With the over volt hack and power diverting/chops, I've hit 45'sh with scary accel on the flat...50 on the downhill. DRY weather pavement scoot only! They are built for heavier riders but are actually pretty light. Speed however comes at a cost, which is the actual range they will run. I get about 10 miles, but its a fun 10 miles!
I have scared a few people who do not know that you're coming up on them at 60 feet a second.
Higher speeds will put you into a whole other price range.. about $3k or more for anything with serious power, real suspension, light kits, real cruise control, etc. Segway makes one with bigger tires (11" I think they are) and it will pull 30mph..could be a good match for what you state you want.
It really all depends on how much $$ you want to toss at it.
If I had to do it over again, for wife & I's RV scoots, I'd go with knobby pneumatic tires no matter what. Lots of RV parks are gravel roads, grass/dirt and what not...the hard rubber wheels are great for dry pavement but have zero traction otherwise. Lack of front suspension can really grind ya down on long rides too and actually makes them less safe than F/S ones. JMHO.