My 5 month old baby often holds his arms straight out like an aeroplane. usually he does this when being held or sat on a knee. is a worry?

No: He is learning how to play and how to hold his arms in various positions. Unless this is the only way he holds his arms it is not a worry. Have fun with him and make airplane noises.

He should reach for,: attain & transfer objects hand-to-hand. When you place him in a sit, he should maintain his balance by propping himself on his forwardly- extended arms, flexing at the waist in a tripod position. His pediatrician can check muscle tone, strength, disappearance of primitive reflexes & presence of protective reflexes and you can call your state's Early Intervention Program to assess him at no cost.