Sounds a bit like ds1 at that age in terms of behaviour
Firstly, agree that it is worth asking for an assessment- ds1 was dismissed in a similar manner at his first assessment, then
as he grew things changed, we saw a different Paed (moved house) and he was diagnosed, now has a statement and is quite clearly Aspergers in the school environment.
My guess is that the confidence issue is a result of other hardships- and they will need to be dealt with separately.
Start with a chat with the SENCO if you ahven't had one, to register your concerns. they can often suggest strategies that can help- things that have worked for us (we ahve four boys, 2 of whom are on the spextrum in different ways)- include designating a named, well chosen older child to initiate play, use of time out systems etc. Have a search for techniques for children on the spectrum- peopleseem to think they are ASD specific but in the main they are just very good problem solving approaches and can work with anyone- two texts that can be used for any child are here (which isnt the dog training book it seems to be LOL- suggested a Mumsnet SALT and found to be excellent), and for practical ideas this book
I am a big believer in instinct myself, so would definitely say that if you feel something is amiss you should follow up.
For the confidence there's a very lentil weavery technique known as love bombing which I ahve used despite its sickly overtones to great effect with all my children. Its very simple- its just about maing an overt attempt to express positive things and state your own love as often as possible in the day. It won'r change the difficulties they face, but IME it does help them do it with an inner glow, IYSWIM?
Oh and welcome to SN mumsnet- you'll very rarely get no response here.