I would be looking for around a $500 midi 88 keyboard or 700 standalone 88 keyboard (the latter in case the laptop would not function properly). It is supposed to be used most times by a 7-year old so it should all work smoothly
Please pardon my delay ---
Given what you want to spend, and that the "target user" is 7 years old:
IMHO you'd to best to buy an "entry-level" 88-weighted-key DP:
. . . . Yamaha P115 (out of production?) / P125 (current) / P105 (older, similar action)
. . . . Casio PX-S1000 /S1100 (previous models PX-150/PX-160) (there's a Sweetwater sale on the PX-770, similar to the PX-150/160
. . . . . . with a minimalist wooden stand)
. . . Kawai ES110 (maybe ES120 has replaced it?)
. . . Roland FP10 / FP30(x)
I don't think there's an obvious "best choice" among those. Any of them will support a kid's musical education through "intermediate level", IMHO.
All of those have MIDI-over-USB, if you want to use them to drive a PC-based virtual piano.
I don't know if they have "Line Out" jacks, but they all have headphone jacks -- usable for driving an outboard amp/loudspeaker system (either "hi-fi" or powered monitors). Plugging something into a headphone jack (on most DP's) mutes the built-in loudspeakers -- no great loss, if the outboard loudspeakers are substantial.
The Casio PX-S1000/S1100 are (I think) the only ones that support "string resonance". The only downside to them is the widely-reported "short pivot", which makes the touch harder as your fingers move toward the fallboard. I think that's something that a player would adapt to. I found it noticeable, when I test-played one, but not upsetting.
Get the basics running first. Then, experiment with virtual pianos, outboard speakers, etc, as money and GAS (gear acquisition syndrome) allows.
Good luck, to you and your son --