I would definitely say it has got something to do with the mould above his bed especially as you say his chest hurts a lot. It could very well be causing a respiratory infection/problem which is causing him pain and making him tired. It might be an idea for him to see a doctor to get himself checked out as it is possible it could be asthma too and the mould is irritating it further. They would then at least be able to give him some medication to help ease the symptoms.

I know you said that his family cannot afford to get the wall fixed, but there are plenty of cleaning products out there that can either kill mould or inhibit its growth. There are also sealants you can buy which reduce the chances of mould growing. I am not sure where you are in the world so I cannot recommend any products by name, but maybe you and/or him could look into what products are out there which might help.

I don't think he has anything contagious though so I don't think you need to stay away from him. If you have the chance to see him on Saturday I would say go for it and just enjoy your time together.