Thank you for your question.
Rest assured that your ex has a very difficult burden in gaining full custody. However, that is not to say that you should not take the petition seriously and engage legal representation if possible.
First, your ex must demonstrate that there has been a substantial change of circumstances has occurred since the last custody order that would warrant the court's review. He may try to use the fact that you have become disabled as the change, but if you can demonstrate that nothing material to custody has changed, the court may not even consider the proposal to change custody.
Secondly, your ex must demonstrate that the proposed modification is in the best interests of your son. In many, if not most cases, disrupting a child's schedule and routine is not in the child's best interests. This can be of even greater importance when dealing with a child with disabilities. You may be able to admit medical opinions that disrupting your son's living arrangements are not in his best interests.
In sum, it is possible that your ex could gain custody, depending on the circumstances. However, he does have a difficult burden in convincing the court that this is appropriate.
I hope this answers your question. Please let me know if you have any follow-up questions, which I am happy to answer. Once you are satisfied, please kindly rate my service, as that is how I am compensated by the website. Thank you.