By age 7, the social ramifications of wearing a diaper are huge. This isn't the sort of thing the other kids at his school haven't noticed. If he hasn't started using the toilet after a couple of months around other kids, this isn't just stubbornness or your failure to win out, something is wrong.
I've only run across a few cases of children this age who aren't using the toilet. Here are the factors that were involved. Please keep in mind that this is not a comprehensive list; it is only those things that I've run into personally.
A child with a medical problem preventing control of their bladder and/or bowels. These kids tend to be extremely ashamed of the problem, and it shows that they are genuinely trying to "be normal" and are hurt by every failure.
A child who has some sort of trauma in his or her life (loss of a parent, ongoing physical or sexual abuse, loss of a sibling, removal to an alternate custody situation, extreme parental neglect, anything on that scale) that has made him or her feel that the whole world is outside his or her control. Children in such situations will sometimes regain their feeling of control and safety by controlling one of the three things they generally can: eating, excreting, or speaking. They may have been perfectly good at all three prior to the trauma, but when they hit the point where they fail to cope, they pick one and adamantly stop eating, stop being toilet trained, or stop speaking until they regain their feeling of control over the world. These kids absolutely own the decision to do what they do, and may actually seem to take pleasure in others being upset by it.
A child who is developmentally disabled in some way, and cannot understand the expectation or its ramifications, but may enjoy the attention they get for lacking independence.
A child who is afraid of someone or something in the general education setting, and has found that by not using the toilet they will get relegated to a smaller special education setting that they think feels safer or somehow preferable, so much so that they are willing to endure the associated status loss at school.
In any of these cases, solving the underlying problem will make toilet training him possible. If you think it sounds more like the medical case, see a pediatrician about it. If he's struggling in other ways to keep up with kids his age, talk to the school about a more in-depth developmental evaluation. If he's had a trauma or may be afraid of something he's not communicating to you, seek counseling.
In the end, of course, it may be something I simply don't have exposure to. If that is the case, I wish you the best of luck and hope that you will return to share what you learn so that the next person facing this problem may benefit from what your family has gone through.