Presumably you mean a private prescription? The chances of getting it on the NHS are fairly unlikely, as you need to be referred to the specialist weight management services which involves long waits and several tiers to go through before possibly being offered surgery or injections, usually Wegovy.
Also, by very overweight, he would need to have a BMI of over 30, or over 27 if he has weight related health conditions eg high BP, high cholesterol, pre-diabetes and so on.
So, assuming privately, you simply choose an online supplier, fill out their consultation form with all his medical details, and they will review and make a decision. There aren't many medications that are contraindicated, and the online consultation process will ask about those and about any other medical conditions.
The website https://monj.co.uk/discount-mounjaro-price-list/ is a price comparison site that lists online pharmacies - they are all registered and regulated by the GPhC. Everyone starts on the 2.5mg dose, takes that for 4 weeks, one injection per week, and then you normally move up to 5mg after that.
Some of the pharmacies offer a subscription service where they automatically re-order for you every 4 weeks, and usually give you the next dose level up unless you actively go into your account and ask them to do something different. Most will wait until you choose to re-order, and will let you decide on the dose level - the same, or one higher/lower usually.
Some places offer additional coaching/support via apps if that's of interest. The comparison site table shows that in a column on the far right called "Add ser/sub" which indicates if they offer additional services or are a subscription model.
I have used Asda and Zava with no issues, I personally avoid the subscription ones and those that offer additional support as I don't want that level of interaction.