I understand your predicament - kind of from the other side, as I don't have kids, but DP does, so I was kind of in your DP's shoes! It's difficult to define a role, and I completely get why you wanted to take things slow and protect your kids - but a year is relatively serious, and it's not fair on him to be your escape or 'part time' partner - if you see a future together, he needs to be a full part of your life... and that inevitably means your kids lives too. It sounds like he's serious about you, but he won't ever be a serious part of your "real-life" until you let him in? And it's the stage in a relationship where you probably would be thinking of next-steps etc, if you are serious about him. From his point of view, with no kids, a year into a new relationship you might be thinking of moving in, etc.
You're obviously some way off that, and moving forward should be 100% on your timescales, but it sounds like he's been pretty respectful of that so far? And his relationship with your kids will develop naturally.
When I first met DP's DS, I would never have wanted to give him into trouble or do anything that would be at all 'parental' - I was very keen to make sure both DP and DSS were comfortable with me and our dynamic, and not step on any toes!! A few years on, we live together, and it's my house too - so DP has been supportive of me doing more chastising where I feel appropriate, and I now feel entirely entitled and justified to do that as he is living with me - though for anything big I still go through DP, and it's taken us a long time to get to that point! Sometimes DP might not agree with me, and those are things we talk about away from DSS - but we are generally a united front, as he realises that's only fair.
I am certain if he felt I was ever out of order or unreasonable with DSS he would speak to me about it, I respect that in the end it's up to him to choose how to parent and I might not agree with it all, so I know my place there - and I know we wouldn't be together if I wasn't respectful and loving to DSS and DSS and I didn't have the good relationship we do have.
It's workable with a bit of compromise on both sides - you have to remember he doesn't have kids, and it is difficult for someone without kids to adjust to a life of what is essentially taking on a parental role, much as you may try to avoid that or even just avoid calling it that! You having kids is already impacting his life, even without him having met them! He might not be able to deal with it, it is hard - but also he may make your mum-life much better / easier as a support system at home, and more romantic interventions in your everyday life?! Sounds like he's earned a chance at it at least.
And if you do end up living together etc, while you're still their mum, he'll be a big part of their upbringing too, and will likely have an input or opinion on things, especially if he's a close part of their lives, that you might not always agree with - but you should learn to listen to respectfully. Conversely, he will need to remember they're not his kids, he's not their dad, and the final say is essentially yours. You can't keep these elements of your life separate forever - though I appreciate why you'd want to try! If you want a serious relationship more than a casual escape, you need to let him in.