Well, an assault causing injury can be made out if a threat to use a weapon is made during the assault.
Your son has to attend that photograph/fingerprint date, if he misses it he'll get another criminal charge.
On his first court date, he should show up early and a parent should go with him. That always helps. He should see duty counsel, get an adjournment and his screening form and crown disclosure, and apply for Legal Aid.
If you want to get him a lawyer ahead of time, you may. But it's not necessary before the first court date.
The threats charge is also an element of the assault charge in this circumstance, I suspect. But you don't want your son to have a criminal record at all of course, so do not let him plead guilty on the first court date. Just get the adjournment, disclosure, screening form, Legal Aid application, and then look into getting a lawyer.
Even if convicted he's unlikely to get anything more serious than probation.
Anything else about this to discuss? Please reply.