First of all - I know NOTHING about mandolins
Second of all - good on you for wanting to do something for your son
Third of all - make sure that it is the right type of mandolin for the type of music he wants to play - bluegrass mando’s are different from the old round-back kind. the one in your link looks more like its for bluegrass.
Fourthly - that seems pretty inexpensive for a mando - you might want to search online for websites that review specific brands. With guitar, there is Harmony Central.com - you have to wade through a lot of bull, but you can tell if someone is posting who know a bit more and you can tell if a given inexpensive brand is still of sufficient quality. I would suspect there is the equivalent for mandolins, too.
Fifthly - whatever you get, right when you get it, take it to a guitar tech (I am sure a decent acoustic shop will work on mandolins, too) and ensure it is well set up - bridge in the right position, action set up correctly etc. Since mandos are 4 pairs of strings (called “courses” - so a 12 string guitar has 6 courses), it is critical that the mandolin can hold its tuning in a stable way - tuning an instrument with courses is a royal pain so if it isn’t stable and drifts out of tune on its own, you’re kinda screwed. Oh, and get an electronic tuner right away (cheap ones are about $20) and ensure your son knows how to use it (the store can show him).
Lastly - I buy from eBay, but always make sure I have a 3-day or weeklong return-for-any-reason agreement with the seller. That way you can take it to a local shop for the tech-check and ensure it is stable tuning-wise and otherwise of sufficient quality. Most shops don’t care where you got the instrument if you are willing to pay the money to get it checked out. They may charge up to $40 - $50 bucks for a thorough going-over, but ideally they will just give it a once-over for free especially if no real adjustments are needed. You should have the shop identified before making your purchase and confirm their terms for bringing an instrument in. If the eBay seller will not agree to a 3-day return, do NOT buy from them - period.
All I got for now…