yes very possible. I know a case friends of my father, the grandfather had it then 2 of his sons out of four. now the ones who didn't get it one has a son who has it, whereas the 2 sons who got it one has a son who has it and he is married but his son didn't.
there are no rules. but what is obvious it runs in families. I remember one of the pdocs i told him i have this BP he replied this is the illness of the honorable people. I am sure one day with the advent of nanotechnology and drug delivery at least to minimize the side effects. we are still in the infancy. how to smooth out both the energy and mood that's the clue. hope soon
Mental illnesses do apear to be genetic but not everyone in a family line may have them or everyone in a line may have them. For example, I know a woman where she has bipolar and her son has bipolar. In my family, however, I have bipolar but it is my 2nd cousin who has schizophrenia, which appears to be a related illness to bipolar. Other people in my family may or may not have things they just don't talk about it.
Also it may not be bipolar but other issues such as bullies at school, or social anxiety / general anxiety, or ADHD and so on, not all being illnesses, of course. Maybe even being gifted and bored. That can also cause problems, oddly enough, other kids may outcast him for it, too.
So, if it is in your family then it is something to watch for. But you should take him to see a child psychiatrist for evaluation. Also talk to him and try to get him to use his words to verbalize. I have a 7 year old also and I think helping him verbalize his feelings is really good for him in general.
I have read that some mental disorder may be hereditary but others are influences by life stresses. For a child is best to seek professional help, like visiting a psychologist to get your child evaluated. The child may be reacting to his surroundings , and may be having difficulty expressing verbally what is troubling and sitting thru psych group therapy, may be worth it.
More importantly should treat this as a separate issue unrelated to your family history until you have your child evaluated by a professional, this enables you to enter into situation with a empty canvas, your past negative experience from the ups and down of growing up around a relative with a mental disorder shouldn't influence your current views on this new onset with your child. Just use it as medical history to share with the professional that will be evaluating your child. Best of lu.