Hi, Kay Marie - I asked my wife Fran about this, who is a PT. She agreed that the numbness is a sign that something is wrong. However she thought it was unlikely that TENS itself, if properly applied, was causing it. She thought it more likely that there is something in his positioning while receiving the TENS which is causing the numbness. So you might ask the PT about that and see if a more comfortable position worked better.
As far as the safety of TENS for PALS, she said that it should be applied at a low level. TENS is supposed to stimulate the sensory nerves and relieve pain. If applied at a high level, it will also stimulate the motor nerves and cause muscle twitching. That should definitely be avoided in
ALS. So if it just causes a tingling sensation that is OK, but if the muscles are twitching and jerking that would be bad.
And Fran also said that you cannot at all assume that the PT would know what to do with
ALS, it is a rare disorder and most PTs will not be familiar with all the details of appropriate treatment. So you should be proactive and take steps so that the treatment is done safely.