If your daughter is not keen on the idea of Switzerland, it sounds like a recipe for disaster. From previous posts, it isn't the easiest relationship so unless your daughter is itching to join you, I would put the idea to her and then let her decide without any pressure.
Children develop at different rates and language acquisition might be delayed if he is unable to socialise because of lockdown. You talk about literacy issues but conversely, the most advanced talkers can have literacy issues too. I think you might be worrying too much on that front. I wonder how you think living with you will change his literacy ability unless you are intending to push him forward in some way. If so, I'd be wary of that as you can do more harm than good whilst having the best intentions.
Landlines (by Raynor Winn, author of The Salt Path)