my dd1 is exactly the same! i have a 'look' that i have mastered and she knows then when she is pushing it and will stop. she is the oldest though so she doesnt have an older sibling to help her but she often 'helps' dd2 take off her clothes and nappies
she helps dd2 because she enjoys helping her and thats probably what your ds is doing. it makes him feel all grown up and protective and he is helping mummy so therefore getting brownie points too.
let your ds help if he offers and at other times just explain to your ds2 that its time to start being a big boy and go ott on the praise to ds1 "oh look ds1 you are getting your own clothes on, well done arent you a big boy you are so helpfull to mummy getting yousrelf dressed" "well done tidying up ds1 you are doing really well mummy is very happy" ds2 will sooon get the picture, but make sure you praise any effort he makes even if it is only as small as putting one bit of lego back in the box.