The thing I found with my DD was that she didn’t like trying to answer questions on computer/app on the times tables if she hadn’t already learned them thoroughly.
It is probably very obvious to say this, but your DS needs to learn his tables thoroughly first. I don’t think it’s effective to learn by using a 'testing' app, as they keep having to work out answers so will be slow or get lots wrong.
With my DD we started with 2, 5 and 10, and when she learned each set (in random order) I’d then test her on random questions from 2 and 5 mixed up, then 2 and 10, then 5 and 10, then all three together. So, this was having them written down, reciting them, her picking random ones, then when she felt ready to be tested, I would test her.
Gradually we added sets, until at the end of that term she could answer any up to 12x12 fluently. Of course, the last few were quick to learn, as most of the answers were covered previously. It took about a term for my DD to learn them all, with me spending about 10-15 minutes each weekday with her.
For testing purposes, we had our own home TTRS account (on computer, not app) which was inexpensive, though not free. I could set individual tables, or combinations, or all tables, for DD to try, and it can test on the division facts which are also important. However, you wouldn’t need to use TTRS if you don’t want to pay for an app/login from home. Once your DS learns his times tables so he is fluent, he’s unlikely to be anxious about answering them in the check.
Anyway, don’t they get six seconds per question? This is masses of time, as once he's learned them he will recall them within a second, leaving five seconds to type the answer and submit it.
For learning times tables, DD’s school used to play the Supermovers songs in class: www.bbc.co.uk/teach/supermovers/times-table-collection/z4vv6v4
One online tool that might be useful is Hit the Button. I think you might have to pay for the app, but you can access the free game on a computer: www.topmarks.co.uk/maths-games/hit-the-button