How do I tell my boyfriend he needs to lose weight?

Within the last few months my boyfriend has been gradually putting on weight...I really don't mind him being a little chubby but he was really thin when we met and he's starting to get boobs now haha. I love him but I don't find him attractive anymore.

He's said he is concerned about his weight but I keep saying it's not a problem and if he's happy so am I. How can I be honest without upsetting him?
How do I tell my boyfriend he needs to lose weight?
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