My boyfriend wants me to tell him his penis is small

My boyfriend and I have been dating for almost a year and a few weeks ago he confessed to me he has a fetish and wants me to tell him his dick is small while we’re having sex but to not make him feel bad. When he told me this I didn’t judge him or really think it was weird but I definitely don’t think he has a small penis. He’s average sized. The next time we were having sex he brought it up and wanted me to say it to him but I didn’t know what to say without it being/sounding offensive so I just didn’t say anything. Well last night I was giving him a hand job and it was going really well and when he was close he kept saying “tell me you want my small dick” and so I said it and after he told me that was one of the best orgasms he’s had. I just want to make it sound more believable from me because I’ll do whatever to satisfy him so what are some other things I can say to turn him on? Do any of your SO want you to say this to them?

UPDATE:

So my boyfriend and I had a really long talk last night about this and everything went great. We even tried some new stuff. Thanks to y’all that suggested advice. This was a awkward/untalked about subject I guess and I just want those of you that commented to know that your advice/suggestions were very appreciated. Have a good day ladies!