Oh dear- it sounds very tough, Flowertop. It's always heartbreaking to think of our kids having a hard time at school, isn't it?
I actually think the most important thing of all is to show Joshua that, no matter what is happening at school and no matter what the teacher thinks of his ability and progress and no matter how hard he is finding it to make friends, you think he is wonderful and really value him for the person he is. I'm sure you already do this in all sorts of ways and this shows in the way you care for him. I do find myself that sometimes I forget to give spoken praise and regularly comment aloud about how great I think my boys are though. So I think that in your situation I would find it helpful, alongside the excellent suggestions here and all the things you are already doing, to try an overt, conscious praise offensive, with the aim of really focussing on and commenting on Joshua's strengths.
I've found this can be very useful, both in terms of making kids feel good about themselves and in helping them to develop, both socially and academically. So if Joshua loves yug-i-oh cards or playstation or watching a certain tv programme or whatever- it might be worth talking to him about who else in his class likes these and trying to find things to praise him for within those interests. Does he know everything there is to know about action man? Tell him so! Can he play video games better than you? Has he learned how to use a computer mouse younger than you learned how to use a pen? (Usually a safe bet, I find!) Is he kind to his brother or to any pets you might have? Equally you might find that book and comic tie ins with the things he really likes will help him switch onto learning more. I know some parents shudder at things like Top Trumps, bland Jackie Chan mini books, Power Rangers comics etc and they are also way overpriced which can be an issue, but they can switch on learning lights, ime. Top Trumps is great for Maths and social skills, for example. I know that it can be hard to find things to praise when it feels like you child is in a bit of a rut but it can be done. At a time when my boy was struggling a bit academically last year and was a bit turned off the whole school work lark I even resorted to praising him for his great sense of humour while he was watching Basil Brush. And even though this felt a bit contrived, when I thought about it it was true- it really gave the whole family great pleasure to see him rofl at Basil and we all ended up joining in because he has an irresistible yukking chuckle. It seemed right to acknowledge out loud that this was a great quality of his. He seemed ever so pleased. Ultimately, I reckon that when our kids are adults neither we nor they are going to care how they did in school when they were 6 or 7 or 12. But they will remember whether they were basically feeling valued and supported and loved. So I try to concentrate on that. Worry levels allowing!
I'm interested in his all singing, all dancing brother too! Is he older or younger? What is their relationship like?