As a teacher I can't express how important is for your child to read and to read often. Every study done under the sun has shown a massive correlation between the amount of time a child reads and their academic success.
However, with that said, your son's homework should be reflective of his ability and should also take into account his learning difficulties. I therefore recommend that you book an appointment with your son's class teacher and go into the meeting with a calm attitude and with the mindset that both you and the teacher will be working together for what's best for your son.
Please don't go in all guns blazing though. We teachers really do want the best for all of our children and I often worry about them long after the school day has finished. We really don't stop and it's not uncommon for us to work 80 hours a week. The record for me is 102 hours thus far, so the last thing we need is a parent making us feel terrible when we're already under enough pressure from senior management and the government.
Go in with the right attitude and I'm sure your son's class teacher will be fair and reasonable. We all want what's best for your son after all. I'd start by asking the teacher how long they expect your son to be working, and then set a timer for your son. You stop the timer when it's up and then you write on his homework this is what he was able to produce in x amount of time and then sign it.
This will give your son's teacher a better idea of what is reasonable and they will be able to make any adjustments if they are necessary.
I was going to recommend you send your son to a homework club but saw that you mentioned that he already attends one. If that is the case, then why isn't he completing his homework there? Is it because he doesn't have enough time or because he chooses not to? Or is it because the staff running the club aren't helpful enough or aren't supervising him enough?
In my experience the staff at clubs are very helpful (I used to run one when I was a Teaching Assistant), but they won't force children who don't want to be there or who don't want to do the work. Does your son want to be at homework club and does he want to do his work?
As for time tables, there are lots of fun ways to learn them. If you look on YouTube there are lots of songs tailored around the tables that are based on popular songs like Meghan Trainor's All about that Bass.