When did you tell your boss?

I’m scared to tell my boss too early, mainly because I don’t want him to treat me differently and give up on me if he knows I’ll be leaving in September. But I’m SO sick with morning sickness, it takes everything out of me to get up for work in the morning. I puke immediately after waking up and feel sick until about 2pm. Not to mention I am so exhausted. We have the ability to work from home, so I kind of want to take advantage of that, the problem is I need to give my boss a reason why. Covid isn’t bad where i live, so I can’t use that excuse. Should I just be honest with him? I’ll be 7 weeks in 2 days.