It is not too late. UCAS just accepts application for the 2013/14 entry. You need not wait until the release of the HKDSE result. I will be too late for you to obtain an offer from top colleges.The registration period is from now to 15 January 2013.
No, you will know the results of your UCAS applications by the end of March. At that times, your son has not yet taken the HKDSE exam. I suppose that the colleges you apply for are those you intend to settle if they give you an admission offer. If you meet the conditions upon the release of your DSE results, there is no need to spend one more year on Form 6 or 7.
If your son want to be admitted to top university, you need to apply through UCAS as the top top tier universities only admit student through UCAS by the deadline (mid Oct for Oxbridge and mid Jan for the rest), and not even in the "clearance" round. After the application, you will get the result (whether conditional offers granted) by March latest and as early as before X'mas, then you can sit back and wait for the DSE result to see if the conditional offers met.
If you wait after the DSE result, the choice of universities and subjects may be limited.