This is a question re my son who is getting Tefilin now for his BM iyH
I am unsure if he is considered left or right handed.
He writes with his left (but writing with his right is not as difficult as the left of most). He cuts with his left.
He will hold heavy items with his right which seems to be stronger physically and will also hold items and open doors etc with his right. He is definitely right “footed”. For some precise work he will use his right, too, such as placing LEGO pieces.
There are various activities which he does with both hands equally (such as brushing teeth)
On which hand should he put his Tefilin?
This is a difficult question and there is a lot of controversy about this. The opinion of the Mishna Berura though is that he would put it on as if he is left handed. As a side point, can he try to train himself to write with his right hand? If he would be able to also write with his right hand with ease, then he would be considered a righty.
O:CH 27-6, M:B 28, Biur Halacha ibid, Igros Moshe O:CH 4-11, Pischei Teshuvos 27 -13,14,15.