In the first place you should be clear that as a 10 year old child, unless he has significant underlying conditions, he really is not at risk of serious illness. Really not. He is at more risk getting into a car than he is from Covid, so please don’t add that to your anxiety.
However, he may very well be at risk at being stuck due to flight cancellations, and you don’t want that!
Unless he is travelling on a US passport it is extremely unlikely that he will be allowed to travel at all because Europeans are basically currently banned from entering the country. If he has an American passport he will be allowed in, but that assumes that his flight goes ahead. My impression is that most flights to and from the US are being cancelled.
If he does have a US passport, and does get in, I do think you risk difficulties getting him back, again due to flight cancellations. And if he does get back he will be subject to quarantine, which may interfere with school restarting or other aspects of family life.
Finally, as an unaccompanied 10 year old their will be other airline rules about paying for an airline guardian, which may very well not be available at the moment. Again, if you can get him there (doubtful) you might not be able to get him back.
Finally, the US is not a signatory to International Rights of the Child Treaty which (for good or ill - not always for good imo) stipulates that the child must be returned to the country where he ordinarily resides in the event of a dispute. Given the difficulties of travel at the moment his father could easily use those difficulties to keep him and you might very well not get back up from US courts.
Bottom line: no way would I send him, not because of covid, but because of the travel difficulties. Let your ex take you to court and see how far that gets him. (Not very far I am guessing, under the circumstances.)