Hi!
The most comfortable accomodation is probably any of the Talybont buildings, as they all have ensuite, in comparison to other accomodations which are shared bathrooms. It is typically a 20 minute walk to campus and the biosciences building, where the course will be based in.
Students tend to really like University Hall as well, which has en-suite options. The only downside is that its quite far, but the university does offer a free bus to campus that operates hourly until a certain time.
Adam Street Gardens is quite nice as well. Its owned by a private company, Unite Students, but sometimes they reserve beds for certain universities. It's very close to the city center, just a few minute walks to the arcades, but is also a 20min-30min walk.
The closest option for the Zoology course would probably be Senghennydd Hall/Court, as it is essentially on the same road as the student union, just a few buildings down! It's right in the middle of campus, but it is shared bathrooms, so it depends if your son would be comfortable with that.
Let me know if you had any other questions!

~ Fatiha, Cardiff University Student Rep