Could my son be behind ?

I know every baby is different and shouldn’t worry so much if my child’s monthly milestones are accurate but I have twins and one is way more advance then the other. I try not to make it a problem but everyone seems to put things out to me. My younger twin still isn’t “stiff” like people say and wobbles around if you don’t hold him in a upright position as to my other boy i can carry him on my hip already without a problem. Also my younger twin does not stand with support, he literally flops down like a noodle to where my other twin has been practicing standing and made it up to 3mins standing up alone. & I’ve also noticed my younger twin isn’t as alert as my other boy, i can call for his attention, rattle toys so he can look my way and he ignores me like if i wasn’t there and my older twin will turn right away. I try not to think about it so much but when everyone always points out “oh he still doesn’t do......” it gets to me. I do have a appointment with our ped next week and will be bringing it up but was anyone else’s babies hitting late milestones ??

(btw they make 6 months Sunday)