You’ll need to try and get feedback from your son, though most likely it will involve some trial and error.
I’d avoid going too cheap or unknown no matter how highly recommended, as those options have little resale value.
Have a look around your local playing venue(s), see which of the following setups seem most popular/common, and buy both rubbers together in the same colour:
Rakza 7 / Rakza 7 Soft
Vega Pro / Vega Europe
Fastarc G1 / C1 (maybe S1, it’s so-so)
V>15 / V>11
Barracuda / Barracuda Big Slam
etc
Test both on the same blade and stick with the one your son prefers and sell off the other one.
For best resale value, try to stay close to 2.0mm. That said, I understand chopping may work better with something under 1.8mm. You’ll need to make an executive decision: save money by having better resale value or “hopefully” go for the more suitable playing experience. I know which way I would choose.
Finally, one awesome advantage of a defensive blade is the larger surface area, which makes it easy to trim down to the standard attacker’s headsize.