I have to say that what you are trying to do is really hard, and in my experience damn near impossible.
I have been married for almost 2 years, and with my husband for almost 4. I have two daughters from a previous marriage, and they are 5 and 7.
My kids are very fond of my husband and try very hard to get his attention, he is not younger than me, and not 21! but he is not their Dad, and he is not very good at paying attention to the girls, he's getting better, but mind you he is their step father, not just my boyfriend, and he has been WATCHING AND LEARNING for 2 years!
I haven't ever really had a "talk" with him to tell him his place, or told him what role he is supposed to play, and I very rarely ask him to take care of my girls. so maybe I am to blame, but he is learning on his own. So in my opinion it works just fine this way.
However the "job description" for a step dad is very different that that of a live in boyfriend of barely 6 months. I would honestly be suprised if your kids could even feel comfortable enough with this guy to want his help with their homework, and if they are, I might even be a little concerned that they are too trusting,(just like their mother) and not to be too crass, but you have been hurt in the past, obviously, so maybe you should be a bit more careful, especially when involving you kids.
I most adamantly feel that you are expecting too much of your new man, he is 21, and way too young to be a father of a 9 year old and 6 year old, think about how old he would have been when they were born!!!! 12??????????????????????????????
15??????????????????????????????
there is a reason he is feeling this way, and saying these things, you guys aren't ready for all this, and neither are your kids.
so my advice is to back the hell up, and take things more slowly.
I know you are thinking that you have already moved him in, and how do you back up now, but you had better do it before your kids get so attatched that they are heartbroken to see him go.