Baby won't sit!!

Natalie

Hey ladies, my baby is 20 weeks old and will not sit unaided. He gets grumpy and pushes his legs into the floor/sofa/bed wherever I try and sit him on his own he HATES!! He loves sitting when being held but as soon as he's on his own he does the pushing thing with his legs so that he gets back into an almost laid down position & cries. Is this normal? Any tips?