It depends on the company you're dealing with. Some 'sponsorship' deals are considered close enough to a sure thing by banks to allow unsecured loans of as much as £70K, on other deals you would be struggling to get more than £35K or so unsecured, usually because there are too many ifs and buts in the contract. Ask how much a bank will lend unsecured, it will tell you how good the deal is.
A really good deal will have a seamless payback system arranged so that, when your son starts work, the debt is subtracted from his salary. If that is the case you don't need a business plan. Loss of licence insurance is a must, as soon as you can get it.
Unfortunately its a father's lot to watch his sons make the same mistakes he did, the young never listen to the old. As my white haired old dad is always saying, "Experience keeps a dear school, but some will learn in no other". For some reason he always glares meaningfully at me when he says it.