This is so sad and it sounds like you have a lot of work to do just to get him to gain some self confidence.
Children pick up pressure from their parents, and it sounds like your husband is really horrible (sorry). Do you really want to be around him and his family? It sounds like they are all bullies. No wonder your child is unhappy.
Until this happens, then the academic stuff isnt going to happen. A happy child will learn, an unhappy one wont.
Learning two languages from bilingual parents should be as natural as learning just one so I wouldnt give up on that.
Do the schools help with regards to identifying problems with children? (think about all the posts on here about assessments etc.) It might be he does have some learning difficulities and these would be apparent no matter what language he speaks. can you speak to the teacher to get a point of view?
What language does your child favour? English or Greek? Is he better with the written skills of one over the other?
To encourage maths, there are lots and lots of games you can play to explain encourage and teach the basics. If you know that he struggles to read the questions, then why is the school giving written questions. You need to understand what the school is doing.
Do both children struggle in the same way?
Parents bragging about what their kids can do happens in every country, you have to learn to ignore it. Some parents dont even realise they can be so hurtful and insulting and dont mean to be, they are just happy their child got good rsults and assume all kids do the same.
Do you use computers to help your children? Sometimes by getting them read computer games, or stories on line can be the motivation to help them read, especially if they wont read books. If you are constantly saying "read this, read that" it will switch them off and they refuse.
I really hope things can get sorted out for you. If you want to ask questions or just talk, there are plenty of people on here who will listen to you.