In Decmeber 2008 I bought a second small property with a buy…

In Decmeber 2008 I bought a second small property with a buy to let morgage on my main property (that I fully owned up until that point). It had to be a buy to let because I do not live in my main property as I currenlty reside in Kent, working in London, returning back home as often as I can. The second property was a venture for my Son to renovate. My idea was to pass on any profits to help him get back on the property ladder and because he being self employed was struggling for a decent income. The idea is good on the face of it but I am now struggling with the tax implications. This second property (and the buy to let mortgae on my main house) is only in my name. My Son is currently tied into a standard mortgage with his Ex-Wife (they too are struggling to sell, presently). As the market is best for rental income, we are about to do this rather than sell, now the property is ready.
On the tax rules, I can not sell the property to my Son at a reduced market value, which is now a lot higher after being renovated. Plus capital gains tax would almost completely deminish any gains even with the £10k allowance, whihch is also the same amount I borrowed on top for the renovation costs.
However, now that the house is to be rented, am I able to pay some if not all of the rental income before tax to my Son, especially as he did all the work for no pay, thus far? This basically is turning out to be a joint business venture, in effect. Me providing the capital outlay, him having labour skills. I am unsure of the most tax efficient way forward in our circumstances (and I am not in this to make money for myself, presently at least).
Thank you for any advise you can offer.
Anita