Really really have to wonder at Jeeves' motives, but i think Bertie reveals it to us in this line:
What was life, without a man to look after you?
Bertie's valuation of self is so tied up with others, tied up with their approval, perhaps Jeeves understands this on some level and won't go all the way with Bertie until his esteem is built up. But it could also be just plain old fear on Jeeves' side. they have defined roles now, even if they're flipped, and if Jeeves moves on to another level of intimacy, they may have to give up those proscribed roles and that may take Jeeves well out of his comfort zone.
That being said.. Bertie is so sweetly seductive I don't see how Jeeves can resist for much longer...