McDowall had previously feared he may not be given a deal by the labels after they chose to sign runner-up Ronan Parke immediately.
The Scottish singer, who will perform at the Royal Variety Show in December, is now signed up to release an album after securing a joint deal.
"Simon is creatively choosing songs for him and is working closely on his Royal Variety Show performance, which will be a great launch pad for him," a source told The Sun.
McDowall picked up 29.1% of the vote in the June finale after impressing the public with his operatic rendition of Josh Groban's 'To Where You Are'.
His debut album is expected to be released in time for Christmas.
> Ronan Parke 'to earn more than Jai McDowall'
> 'Britain's Got Talent' Jai McDowall still has no record deal
> Jai McDowall wins 'Britain's Got Talent'