Do you mean a keyboard, or a digital piano? I assume the latter? A keyboard does not have weighted keys and as such, DS will not get a "feel" for what a piano is really like to play. Which will mean he may not be able to transfer performance skills to an piano (be it digital or acoustic). A good digital piano is almost the same as an acoustic one... you're not meant to say that because folk are very snobby, but it is true.
Also what are your goals for DS music education?
If you literally want him to be a concert pianist, I suppose an acoustic piano is slightly better than a bog standard digital piano - but it's like the difference between bf and bottle fed, it's a 0.5% sort of difference... not worth the spend at all.
If you want him to have a general education in music, then there is no need to get him an acoustic piano. I would stick with said digital piano with weighted keys.
Acoustic pianos are a pain to move, get out of tune without you realizing it which can compromise his ear over time, are expensive, can be fucked up by a kid dropping something into one etc. Digital pianos are great.
Source: I was educated in music using an acoustic piano. My DC use a digital piano which, imo, is nicer to play than the acoustic I learned on, with more clarity of sound and evenly weighted keys etc.