Ideally you’d wait closer to 6 months because it allows babies gut etc to develop more to be able to process food. However, every baby is different and needs this at different times. If you feel baby needs it then I’d go with, if you’re not feeling he’s ready then I’d wait. I know it’s a hard one but you know baby best x
I've always heard/read that baby rice has little nutritional value and gets them used to sweet tasting things. I started with mushed up veg at 5 months mixed with some breast milk with my first and then did baby led weaning when he was 6 months x
My partners god mother heads up the health visiting team for Berkshire and she’s said that they actually find more babies with allergies if you wait to start weaning. If your little one is ready and it’s been recommended I wouldn’t worry, just keep an eye as most little ones gag when they first have something more solid x