No "accept" button shows up on an event I've recieved

I have a yahoo.com account linked to iphone mail. and a diffenrent account for Apple ID. so I have three different calendars. no.1 is from yahoo, work and home are from icloud. my boyfriend has an hotmail.com account which is linked to iphone but not an Apple ID account just the same as my situation. Here is the issue. So if i send him an event through his hotmail accout, there will be "accept" etc. buttons show up when he open it. but if he send me one. i don't see the buttons at all.

hope someone could help to fix this problem. I'm getting frustrated now.

Thank you in advance!