I’m very sorry to hear about your difficult situation. It is a good thing that your in-laws are under treatment for their conditions. Regarding your husband, because there is a family history of mental illness, he has a genetic vulnerability as well as difficult upbringing which may lead to future possibility of mental illness in him as well and unfortunately it makes your child vulnerable too. However since your son is very young, you can take proactive steps to give him a strong personality to protect him from future stressors. If he suffers from stranger anxiety it may right now just be natural for his age. Give him an emotionally secure environment, a sense of stability in daily routine, make sure that he understands that his grandparents are ill but don’t let the stigma or the distress affect him negatively. Try to control your temper when you’re with him and let him know that in you he has a strong support that will never fluctuate.
I wish you all the best.