Ok, Late post, I know, but... If you already include toys in your sexual relationship, make it clear that this is simply a toy rather than a replacement of his penis. The title "penis extension" is a rather insensitive term... think of them as dildos your guy can control like his actual penis - a metaphorical extension of his penis, like a tool, or in this case a dildo can be an extension of his hand. Explain to him that what's lacking with other dildos is the intamacy you have when he's inside of you, etc. It's not about replacing him, it's about using a toy (for variety, adventure, challenge, etc) that's more intimate, more real.