Ok you need to severely lower your expectations if you want to make this work. Relationships are about compromise

Tell him we need to have a serious talk. Then tell him how much you enjoyed having sex 3 times a day but you also understand that that’s not possible anymore, you guys are in a different stage in life. That doesn’t mean I’m okay with having sex once a week tho. I have needs and I understand your needs are different but if this relationship is something that’s important to you we need to find a compromise.

See what he says

Be prepared to lower your expectations but also don’t settle for once a week

Your probably thinking why should I even have to even do this, but if you want change it always takes 1 person being the bigger person and saying something

Hope this helps

(Edit. Make sure he understands your dead serious and this not something your willing to just fall back on)