How old is the bird? when birds are parent fed, they parents force the food into the baby's crop in a bobbing motion, then hand feeders use syringes to accomplish basically the same effect , so trying to eat from other birds' mouths is most likely a begging motion & would understandably get mad if they don't feed it.....if you're talking about pulling your clothes into his mouth and sucking, that could also indicate it wants to be fed.....some young birds will regress some when taken from their siblings to a new home, so getting some hand feeding mix & following the instructions may make it happy.....
Good luck.....