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I am very sorry to learn of this situation.
Unfortunately your nice landlord needs to become involved - it is his responsibility to work with the HOA to ensure that your rights are not being violated (your son has a right to live in the unit and be undisturbed on the basis of his disability, the Association must make "reasonable accomodations" for any issues which he may have).
If your son is a "heavy walker" they must make reasonable accomodations - this would certainly include provisions to protect your son against harassment from other neighbors.
In the HOA framework, the way to enforce these rights is by an owner (so your landlord) pursuing a claim either directly against the offending owners - or more commonly - against the HOA itself, and require the HOA to deal with the matter.
Short of filing a suit, mediation with the Association may be a worthwhile first step to have your landlord get them involved in resolving this situation.
If nobody steps up, sue your landlord for breach of contract.