Grandchild not allowed to visit my home

I'd feel the same as you. I'd be heartbroken and hearing he visited the maternal side of the family I expect I'd feel exactly as you do. I think you could tear yourself apart looking for the reason but honestly it'll do no good you may never really find the answer. All you can do is be the best nanna you can when you do see him. Put on a brave face and bear the pain in private and of course with us. The other route could tear your family apart setting sibling against sibling. Handled sensitively things might improve over time. I hope they do

Unjustified though it might be, perhaps your son wants to protect his child from what could be viewed, wrongly or otherwise, as obsessive cleaning behaviour. In other words, it may not really be about third hand smoke.

Whatever the reality of the case, I think you just have to accept his decision. So long as you are able to visit your grandchild at his own home (or perhaps to meet up with your son's family elsewhere), you are getting the important part: seeing your grandchild. Whether you see him in your own home or somewhere else is really a small detail.

Everything jane said.

I would not go down the route of looking for evidence that your son's ruling is an 'excuse'.
You are bound to think you have found some and that could be the start of the rocky road to estrangement.
I'm afraid you will have to accept your son's reasons at their face value, and build a relationship with your grandson within those limitations. Babysit him at their house, or take him out to places.

The really important question is why doesn't your husband stop smoking? His health would improve, and you wouldn't have to do all that tiresome housework.

I would look out for evidence that they do not carry this through with other people.

But if they do, it may not be an excuse.

I wonder whether, do to your compromised immune system, that your son is super alert to all things medical.

Do you have any idea why your son would be making an excuse, if smoking is not the real reason?