Coco1
"Going away too much" can't possibly be a reason for not answering a text. We can text anyone wherever they are. Currently I'm in close contact with my friend by text as she is visiting her son in the US. I normally have frequent telephone contact when she is at home in another part of Britain.
I think he is making an excuse to his sister and saying the first thing that comes into his head.
I do think too, that given his previous illness, he may be inclined to live differently to the rest of us and not have the same reciprocal relationship behaviour which most of us take for granted. Obviously I don't know his condition and even if I did, I don't know him, so it could be any number of reasons why he is so uncommunicative.
I would try not to read too much into it, try to wean yourself away from worrying about him. I realise that is hard, having been so close to him and done so much for him to get him through his illness. I wouldn't pay much attention to the excuse for not being in touch either. Maybe just send him a message once a week saying how you are and sending your love and asking if there's anything he's like you to do for him. Actually maybe not that last bit - you can decide if it is appropriate. You can invite him over for Sunday lunch or something. It isn't against the rules to get cross if he persistently does not reply, so long as you have ascertained that your texts have arrived!
It could be that he feels - even without actually putting it into words - that having received so much help from you at a very bad time in his life, he now finds it hard to see you. He may find that it brings back that time, or he is embarrassed about how he was at that time. I remember when I was a Psychologist, walking through the Hospital with a Psychiatrist friend. We passed a teenage girl to whom my friend said hello, but the girl turned away. My friend said she had treated her for anorexia nervosa and the girl had been seriously ill. She said she understood why the girl did not want to be reminded of that time or to talk to the person who knew all about her at her worst. It made a very big impression on me. Then my husband died by suicide. We had the most amazing Funeral Director, whom the Police suggested. I always planned to go back to the Funeral Director's premises to just visit him a few weeks after, as he had said I would be welcome to do. Whenever I drove in that direction, I began to feel colder and colder and as if I would be sick. I couldn't go back to that time and he belonged to that time.
I am sorry to say you can't do more than you have done. I think Smileymiley7 has said it perfectly. If you can meet up with him this would be a good idea. But do so without any resentments, just be amicable. You probably have to adjust to his personality being such that he is rather incommunicative. Some people are. But just keep a simple, unprovocative message going to him about every week or even once a month. I say that in view of his having had a mental illness in the past. It is good that he can always make contact with you and know where you are.
Good luck. I know this is very difficult. I hope you get some good news soon so that you can relax and leave him to it.