Seren went throught his stage. When she first asked I said "no" really quickly and immediately felt bad - I had said I would never said no but I felt quite protective over Cally's food I guess. I spoke with OH and decided if she asked again I would let her. She did ask and I said she could after Cally had fed, by which time she had forgotten but she did ask a number of times and a couple of them has tried to latch on. She has completely forgotten how to latch on (I was so suprised she had forgotten so quickly) and just put her mouth on my boob, then laughed and ran off. She still has days when she ask and normally just kisses my boob as Cally is feeding or just snuggles into my chest.
It is pefectly normal, toddlers regress slightly when a new baby arrives and it is very normal for them to do "baby" things. Seren asks to be carried like a baby (either cradled or in a sling lol), have her appy changed even though she is in pants and uses a toilet (she also weed herself a few times) etc etc. Other days she is a big girl and wants to be all independant. Go with whatever makes you feel comofrtable, you may find he has a few feeds and then stops, or tries and gives up, or just wants you to say yes for some kind of confirmation he is still your baby. I found cuddling Seren with my free arm when feeding Cally really helped her to feel included, and we still do this now but its not as cute as Cally wants to bash her big sister and Seren wants to kiss her lol.