Look he already knows what it is, and because of that he needs some sort of validation. Lying to him is a bad idea, he'll see right through it. The truth is a worse idea, unless you want him to lose any confidence he's ever built up and hate himself. So what do you do?

When people talk about size, they usually are referring to length. Your best bet is to flip the script on him. When he asks about size, focus on an aspect of his genitals other than length. It could be about how thick he is, how big his balls are, whatever. As long as he walks away thinking there's something about his genitals that you really like, it'll be an emotional win.

As far as using dildos goes, don't even go there right now. No matter when it happens, it'll probably hurt him, so that shit needs to wait. I literally don't have words for how bad of an idea this is. How do you think he's going to take it if the conversation is essentially "you're not too small, but oh by the way I want to bring in a substitute for you that's bigger, but it's not because you're too small, I swear?"

When you do cross that road, and just know it will most likely hurt him regardless of whether or not he shows it, do what you can to minimize the notion of it being a replacement. Don't get one that's super realistic with veins or balls if you can help it, and try to go with a non-human color like green or orange as opposed to a skin tone.