I can already tell that your client's ALS is manifesting differently than it did for my client. My client knew she couldn't take deep pressure. Her concern was that I wouldn't be able to work light enough to be comfortable. (Her husband tried to massage her as light as possible and his pressure was still too strong). She would never have requested deep tissue therapy.
Every client's different, even when diagnosed with the same disease. This is a great example. My recommendation would be to begin moderately to warm up the muscles like you always do, and then check in carefully as you increase pressure. If he says the pressure feels good, I would work deeply as requested. However, I don't know if this is an option, but I would recommend a "trial session" for deep work -- 15-30 minutes for the first visit. Then call the next day to see if there was/is any after-massage soreness. Sometimes people want the pressure because that's what they preferred when they were healthy. They don't expect any after-effects, but when they are seriously ill, their body may respond negatively. They wouldn't know this ahead of time (unless they have received deep tissue therapy recently from another massage therapist). If when you call, he's feeling great from the deep work, you can schedule another appointment and do a longer session. If he's hurting from the massage, you'll know to work lighter at the next appointment.
That's my opinion, anyway. I would be interested in hearing how the appointment goes.