To fit a refurbished engine takes only a day or two. To (attempt to) repair an engine that's been run without oil might take a few weeks (especially if some grinding work, for example, has to be outsourced) but:
(a) it definitely doesn't take 6 months ; and
(b) it would usually be far simpler and cheaper (as well as far more likely to yield a successful result) to put a secondhand engine in anyway.
However it does occur to me that your son might have suffered the same unfortunate circumstance that happened to me. I was regularly checking the oil on my Modeo, and finding plenty on the end of the dipstick, but unaware that there was a blockage at the bottom of the dipstick channel. As a result, I ended up with no oil inthe engine (which was wrecked beyond repair) but still plenty on the end of the dipstick!
You've posted in 'Law' and a 'legal' response might be to send a formal written demand to the trader for a refund of the money that was paid for the car, threatening legal action if no refund was received within a specified period. Then, assuming that no refund is received, initiate action via the online small claims procedure. If your son wants to follow that route, my post here explains what needs to be done:
http://www.theanswerbank.co.uk/Law/Question1076103.html
(That's a totally different legal dispute but the procedure is exactly the same).
However, in the first instance, your son might find it better to contact the Trading Standards office covering the area where the trader is based:
https://www.gov.uk/find-local-trading-standards-office
Their involvement might spur the trader to action.