Hi, you probably know that antidepressant medication can take 6 weeks or longer to kick in. In the early stages, the depression can feel worse, the patient may feel like sleeping all the time. Your son needs to talk things through with his GP. If he's been diagnosed with severe depression, it's likely his GP has discussed a referral for talking therapy.
There are often very long waiting lists. CBT is the governments choice, they have some magical notion that 6 weeks of CBT is a cure all. Some research confirms that it's good for phobias and 6 sessions may indeed help the patient to get on the plane, begin the process of dealing with anxiety triggered by certain events.
I'm not dismissing its benefits but for some people, longer term psychotherapy is needed. Usually, it seems folks are referred for CBT and if necessary, go back to their GP and ask for a referral to adult psychology for longer term therapy.
There are some good private therapists. Expensive and not all therapists are as skilled or well qualified. The internet can be helpful in finding people who are professionally qualified and registered.
Best of luck to your son and to you