Is he not into me anymore?

Me and my boyfriend have known eachother for 3+ years & we never really had any problems being intimate but lately for a month or 2 he’s been turning me down & I’ve gotten upset about it thinking he might be seeing someone else or thinking about someone else. He said he’s changed he doesn’t know what’s wrong he looked it up & said it could be because of his diet but I just can’t tell if he’s being honest.

Is he not into me anymore?
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