how to start that conversation!👪

how do I bring up the conversation to my boyfriend that I would like to start trying to have a baby??? his always been very confusing about this when we first got together it was like 22/23 I would like a baby and now his 23 it’s like oh I don’t know and then sometimes it’s ‘I don’t want children at all’ and then other times it’s ‘I want a family I would love to be a dad’ I haven’t mentioned anything for a while because sadly his mum not been about for a while and we have taken on (adopted) his 10 year old brother so I didn’t really know how to bring it up because all his time and effort is looking after his brother obviously, but how would any of you bring it up if you were in my shoes? because at this point I feel like I’m never going to be a mum and it’s scaring the hell out of me especially not knowing how long it will take to get pregnant and if there will be any problems and if we can actually have children and I would like a baby by 23/24 latest I’m 21, any advice??