My SO wheres the wrong size pants.. How can i talk to him about it?

I know very well that my partner is very insecure about his body. Whenever we go clothes shopping i try and get him to get the bigger size but he just wont. He'll try on the bigger size and then say that theyre too big and get the size he thinks fits... The issue here is that he's not understanding where pants should sit on his body. He is not sagging at all. Hes wearing the pants so that theyre up in the front but in the back are only at about the half mark on his butt crack. So when he even slightly moves or bends over his butt is there for the whole world to see. All it would take is for him to wear the next size up so that he could actually pull his pants up to where theyre supposed to be. 
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To help this make more sense: he is pulling his pants up all the way, NOT SAGGING. But because theyre too small they only go up so far and only cover his butt half way. If he bought a bigger size there wouldnt be this problem. 
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He wont listen to me about it though and continues to buy the smaller sized pants that dont fit properly. His butt continues to be on display. And he gets rips in his pants often because they are too small.
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I try and tell him to try the bigger size. But I havent come out straight and told him "everyone can see your whole butt when you even slightly bend over or sit down" because I know how incredibly insecure he is about this. And I know it would hurt his feelings for ME of all people to be saying something. 
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Ik "if you truly loved him youd say something". I love him to the end of the universe and back!!! Hes my soul mate!! My everything!! I want to tell him in the nicest way possible though, i dont want to hurt my sweethearts feelings. And i know if I say it wrong it will incredibly damage him. Hes extremely insecure