I have vans on distribution work for years but never touched parcel work as the rates as crap. As from talking to lots of ex couriers it seems new guys get the shit routes with lots of distance between drops or lots of unavoidable parking tickets etc. Experienced couriers moving firms get nicer work and good paying jobs early on but then start to get more shit as someone else new joins. In the old days the best way to stay on a profitable route was to backhand somebody and I dont suspect that has changed.
I have never leased new vans either as vans are built down to a budget and designed to start falling apart within 10 years, I generally buy 2-3 yr old, off someone that just has to have a new van, and lease them to my guys for about half the cost of a new van lease.
I dont know about UK mail but I believe most parcel firms will let you use your own van as long as it is plain and unsignwritten. So I would definately recommend he looks for a cheaper van option so that he can pay it even on the quiet weeks and, should he find the work is not to his liking, get out easily.
Remember not all accountants are the same, an accountant that is good for a business turning over 2 million wont be any good for a self employed courier. I would ask around the other couriers and see who they use / recommend and what they pay.
My opinion would be dont touch courier work, but if you have to. Try to do it on the cheap until you have a year or more's experience of what your earnings are ,truthfully, likely to be (take the numbers they give you and half them)
Finally look at City Sprint as an example. The parent company pulls a few million out and lets it go tits up. They write any loss off against tax and some of the directors probably have a few quid tucked away.
All the franchises get knocked and wont get anything back. They are probably fucked over for a few hundred grand each. As are the fuel suppiers, van suppiers, tech suppliers (although some of these will recoup a little by chasing the individual courier drivers)
They in turn knock all the couriers, self employed lads like your son that are now potless over christmas and start the new year by having vans repossessed and still owing on them. At least, if you own your van you only get knocked wages and fuel bill.