When did this start and how long has it been going on for? What is his speech like? Could he be struggling to communicate and feeling frustrated? Has anything happened that might of upset him? Do you think he could be using this to try and get some attention as he has a twin and am sure that can be a challenge to provide equal one to one. Sorry for lots of questions but I feel they are all things for you to consider.

He is at a very challenging stage where he is going from toddler to very small child. So much is going on inside that brian and this could be his release. Being aware of sen is good but also I think lots of small people do this as a release for some reason.

My own daughter used to smack her head off walls, floor, bed and me if she could. She would completely lose it. Which was so hard as she appeared so happy and content most of the time. I had to buy her a bed with a padded headboard because I knew she would head butt it. First thing when bed was set up she smacked her head off it to see how hard she could do it (this happened between 2 and 3 years). She is now 5 and all this has stopped and you would never of thought she would of done it. Still keeping eye open for additional needs especially with recent attention around autism and adhd being different in girls to boys.

It is your call if you want to chat with health visitor and maybe get some support. It could be he is simply a small person having big feelings but even this can be really hard on us parents and asking for help and support is never wrong.

How is his speech and his development generally? Biting, frustration, tantrum, etc while difficult to deal with are all developmentally normal, particularly if he is having difficulty expressing himself.

Are there any triggers you've noticed? One of mine used to bite when they were tired or annoyed so I learned to pre-empt it with distraction, a cuddle so they'd sleep, and things like that. They did grow out of it.

When he's angry in the buggy is it because he wants to get out? You could try some backpack reins and let him walk beside you and his sister in the buggy or maybe he'd prefer to stand on a buggy board (with reins on also in case of bolting).