As you may be aware the new drug driving provisions came into effect last year and now mean that there is a specific limit for drug concentrations, similar to the drink driving limit. For cannabis the limit is 2.0 nanograms so your sons reading has come back just over the limit. This offence carries a minimum disqualification of 12 months, although as your son's reading is so low I would not expect the Court to go much higher than this.
Even though your son has held his licence less than 2 years I would not expect him to be ordered to carry out a re-test after his disqualification ends. This only kicks in automatically where his licence is revoked for receiving 6 penalty points in the first 2 years of passing his test. In all other cases it is at the discretion of the Court as to whether they wish to order him to take a re-test, in similar drink driving cases they rarely exercise this option.