Closed Off Behavior from Partner

Hi guys, need some advice regarding some worrying behavior from my partner. (For context, I live with him half of the week, and the other time I am away for work at a work accommodation to cut down a long commute.) On Tuesday, I noticed that he hasn’t responded to me in over a day which is very unlike him. As I have been sick, I hadn’t seen him in a week but everything was fine during that time, still talking normally. I try to call him a few times and it goes straight to answer phone, I don’t think much of it and wait till later. By late evening he still hasn’t responded, so I start to get very concerned that something has happened to him so I make the long journey home to check he is okay. He’s in bed fast asleep. Turns out everything was fine, but he’d had a terrible week at work which had finally hit him on Tuesday evening and he’d turned his phone off and slept solidly all day. I told him that while I fully appreciated and understood that he must be exhausted, his out-of-character behavior made me very worried and upset. He apologized and said he didn’t mean to scare me, everything was just very overwhelming. He said he was also feeling a bit unwell, like he was coming down with something and that he was very worried about an important meeting he was going to have with his boss. I was understanding, and we seemed to have had a good talk. I then stay at home for a few days and leave Thursday afternoon while he is at work and head back to my work apartment to run some errands. As I had planned some time off work for the holidays, we had planned for me to come back the next day on Friday so I could spend the weekend (he was going to pick me up). On Friday, he messages me at 12 noon saying that he feels like he has come down sick with something bad and thinks it’s best I don’t come over. Completely fair. However, he has been completely M.I.A since then. I have tried to call him multiple times as I need to discuss some Christmas plans (not to mention checking in to see how he is feeling) and he will not pick up his phone. I have messaged him on Facebook asking him politely to call me when he feels well enough, but he hasn’t even looked at the messages. We have gone days without speaking before, it’s not unusual and we have a very loving and good relationship. When I was home he was clearly tired and stressed but still very loving and normal otherwise. However, the reason that this is so concerning to me is that this is extremely out of character for him to just not respond to messages and calls. It’s particularly upsetting as we had an open honest conversation about how him going M.I.A was very worrying and upsetting for me and that I would have appreciated a message to let me know he was going to switch off for a while. To clarify, he is a fantastic partner and a good man, therefore this lack of communication isn’t normal for him. I’m concerned that something has happened at work that I’m not aware of and he’s not willing to discuss it. I haven’t reached out today, as I felt it was best to give him some space and don’t want to hound him with more calls if he wants to just disconnect. However, if he doesn’t check in by tomorrow I feel like it will be necessary for me to go home and see what is going on. I don’t mind if he doesn’t want to discuss the issue in detail, but I feel like he is shutting me out and I have no idea why. What is the best course of action here? I don’t want to invade his space but I’m really very worried about him. How can I communicate to him that I am there for him if he needs it, while still giving him healthy space?