Help Boyfriend hints towards a threesome

Okay so my boyfriend and I have been dating for 4 years. Were both sexually active. I don’t really know how I feel about it. I’m slightly open but then again I don’t know how he will feel afterwards. I’m almost 21 and he’s 22 now and we’ve talked about the future and everything and tend to be with each other for a long time. He sent me this text last night after I went to sleep. Any thoughts or tips on how to talk to him about it? I’m usually a pretty open person but I don’t want to hurt his feeling and such. Like am I not enough for him?