Playing a musical instrument is always a good thing, so lets see what we can do to help.
First of all, how old is your son? If he's very young then a 3/4 sized instrument could be ideal, but there's a lot to be said for starting him off with a full size electric if he's big enough (or soon will be).
Although the neck seems very long for short arms, you can easily place a capo at the 5th or 6th fret. That would reduce the scale length to around that of a 3/4 size instrument so fingering is easier for small hands. But as he grows up and gets more experienced he can move the capo back a fret at a time until he is playing the full scale.
The benefits are that you have a lot more choice of quality starter guitars, and you don't have to buy a new guitar in a year or two when he's sprouted so much that he's taller than you!!
Whether you are buying a 3/4 or full scale guitar, it's always best to take a guitar playing friend along with you. Although there are some reputable makes, when you are at the entry level there can be a lot of variation between individual guitars. The only way to get a good one is to play a few, so take your tame expert
I'm sure they'll be more advice to come
When the going gets weird, the Weird turn Pro.