For driving whilst under the influence of drugs, assuming marijuana is the only drug they found and there was no alcohol, this will be a category 2 or category 3 offence depending on how much it was affecting his driving. The minimum he will get is a fine and 12 months disqualification, the maximum is a high level community order and 28 months disqualification. Where it will fall within this range depends on which category the offence falls into and any aggravating and mitigating factors present.
If he is also prosecuted for driving without insurance, assuming he wasn't involved in an accident, he can expect a fine and 6-8 points.
I'm assuming his brother won't report him for taking the car without consent. If he does, that will add to the penalty.
Pleading guilty at the earliest opportunity and showing remorse will help to minimise the sentence. You may want to consider employing a solicitor to plead mitigation for him.
If he is convicted, these offences will appear on any DBS checks for 11 years. However, an employer should only refuse to employ him if the offences are relevant to the job. If the offences are not relevant, an employer would be breaking the law if they refused to employ him. I'm sure some employers do break the law in this regard, but many understand the rules and comply with them.
If a role does not require a DBS check, he will only have to disclose these offences if specifically asked. Contrary to what is said by another poster, this does not depend on the offence. It depends on the sentence. A disqualification is spent when the disqualification is over. A community service is spent when the period of community service is over or, if the order does not specify an end date, 2 years from the date of conviction. As with DBS checks, an employer can only legally refuse to employ him on the basis of these offences if they are relevant to the job.