As a former Kindergarten teacher, and as a mom, I have seen this happen often! Depending on the age of the child (the younger they are, especially boys, the harder school can be), he may not be emotionally mature enough to go on to the next grade. Sometimes, they can do some of the work, but not all. If promoted to the next grade anyway, some students continue to struggle throughout the years, when they didn't get the remediation they needed at the time. ESPECIALLY if reading is the problem. A lot of times, advancing a child, hoping he will catch up, has the opposite effect and damages his self-esteem when he sees that almost everyone around him reads so much better and/or makes much higher grades. This really starts to show up in 3rd grade (or whichever year they start getting letter/number grades instead of evaluations). My own son, who is almost always smarter than most kids in his class, started the thing in 3rd grade about not doing the work at all or not turning it in. His grades plummeted! He had to work at it, some was organization skills, attention problems, and he still struggles with it a lot of times (now in 6th grade). My daughter on the other hand, is one of the youngest in her class, loves school (mostly because of the socializing!), but struggles in 3rd grade now, even though she is a great reader. She dislikes Math and fights against doing it. Sometimes she goes through cycles of not doing her work at school and wanting to bring it all home for me to "help her". After a couple of weeks and stern consequences, she usually shapes up for another few months and tries harder.
Most of what he is going through is based on personality, some of it is a stage common in most children. But how his parents and his teachers deal with it, that will determine how it affects his future as a student. There are still a few months left to really work with him, both at home and at school. You are on the right track by having an open relationship with the school, teachers, and getting him all the help they can provide. Some other suggestions would be, if at all possible, to try to schedule time to come in to school and have lunch with him, spend time in the class helping the teacher or other students, or doing fun activities with the class. I see that you work and have smaller children too, but whatever you can do, even if only sporadically, will show him how much you value education and that school can be fun for you too! Also, if there is any older child he looks up to - a family member or friend's child or a neighborhood kid - try seeing if that child could help out. Older children usually love to help a younger one with schoolwork, or just talking to them about how much they love school and which subjects they like and even don't like. He needs to understand that he isn't going to LOVE everything about school, but that everyone has to go and do their best work. And talk to him about when you were his age, and have his dad, grandparents, whomever... also talk with him about those times in their lives. Stress the value of education and that you didn't get to be in the job you are by just being lazy in school - you had to work hard and get good grades and keep trying when you didn't understand something.
Eventually, all this will sink in. Keep reading - all the time - let him see older family members reading as much as possible and DISCUSSING what is being read is VERY important! Some kids can read the words on a page, but not understand what they read about. If it comes down to it, I am a firm believer in retaining children in the earliest grade possible. I now wish I had held my daughter back from starting kindergarten when she was just turning 5 - but she seemed ready at the time. This is the first year she has struggled much at all. She is working through it okay, but if she shows that she isn't ready for 4th grade by June, she will be back in 3rd grade again and ready to mature at her own pace.
Sorry this is so long, and I am sure you are getting lots of other great advice. Heed whatever makes sense to you, and just remember that you are doing the most important part - and that is being involved! You'd be surprised at how many parents don't show any concern about their child's education or school behavior.
Best wishes and God bless!
A. V