Sounds like a fun father/son project! The answer largely depends on what you want to mine. If you want to stay with the X11 algorithm, you might consider nVidia cards or ASIC. I personally have never tried to mine with any nVidia cards or ASIC but read that they do better with X11 as compared to Radeon cards. If you plan to mine Ethereum, then this write up is applicable. I'd suggest buying RX500 series cards (RX400 series are slowly phasing out) and go for the brand that is on sale, I bought the Gigabyte RX580 for $219 from Newegg.
Have a wonderful time building the miner with your son!
RE: Basic Guide: Mining Ethereum with RX400/RX500 GPUs