My son confessed that he feels scared all the time in the class so we're looking for a new dojo, hopefully with specific beginners classes. Thanks everyone.
Sounds like the right decision to leave.
I don't know of any dojos (around here) that have specific beginner classes for kids. The concensus is that it is better to devide the kids in groups according to age than according to level - one dojo has four different age groups for kids, including one for young teenagers. And I don't necessarily agree that specific beginners classes is the clue to beginner friendly training. The most beginner-friendly dojo I know of doesn't have beginner classes for adults either but "basics classes", populated by people who train their first second and third term but also by others, including black belts polishing on their basics. Beginner training with beginner easily learn bad habits. Training with a black belt every now and then can be an incredible help. And no, they don't find it a bore. If they do, they don't go to basics classes (or they don't train at this dojo)...
But finding a more beginner friendly dojo sounds like the right decision for both of you. (If your son wants to continue doing aikido.) Perhaps looking at a beginner class and check if the atmosphere is relaxed or tense is a good start to evaluating the next possible dojo.