My 2 yrs old boy a至z都識讀 (both small & block letters),0至2X都認得 (in Chinese & English). So, I think the problem of your son is not related to 細仔.
Perhaps you try to read more books/labels/bus no./floor no. with him. My son will tell me the floor no. everytime when the lift door is opened and tell me alll numbers & alphabets he saw on the street/train.
And for me, I started reading book with my daughter (born in 1999) since she was 2 yrs old & asked her to repeat the name of the book once after me. Now, her teacher also says that my girl knows more Chinese words than other kids.