I'm so sorry to learn you're a member of the club that no-one wants to join.
Have you checked out the Tamoxigang thread on the General Health board? A number of responders have been through the ordeal that is bowel cancer and may have suggestions for overcoming the residual health issues you've been left with - one of the regular posters, amberlight, is particularly knowledgeable as she crunches the stats and the latest research and presents it in refreshingly jargon-free manner.
With regard to your son, I would suggest that you sit him down and have a very frank talk with him. The hormone driven years before maturity can cause teens to become completely self-absorbed and selfish, but you need both of your men to sort out their differences, or put them to one side, because you need both of them in your home and it's vitally important that your home is a stress-free zone.
As your ds becomes increasingly involved with living his own life, he'll spend less time in your company and he should understand that it's not fair for you to have to spend hours alone when there's no need for you do so. By the same token, you don't want him spending time away from his home because of any animosity he may feel towards your bf.
Just as the love that you feel for your bf cannot be compared to that which you feel for your ds, the support you get from your bf is markedly different from that which you gain from your ds and there's no need for either of them to compete with each other to be the 'most helpful' to you.
Hopefully, your ds will respond positively to any appeal you make to his better nature and it's also to be hoped that your bf will demonstrate that he's the bigger man by actively seeking a rapprochement with him based on the love they both have for you.
for you with best wishes for your wedding day and hoping that your ds can fulfil the dual role of walking you down the aisle and best man to your bf - and that he's the first to make a toast to the happy couple 
As an aside and in common with the advice I would give to anyone who has dc and is contemplating wedlock, may I suggest that you review whatever provision you have made for your ds in the event of your demise and seek legal advice if necessary as any Will you may have made will be invalidated by marriage and your spouse may inherit a large proportion of your estate should you die intestate. This would be a good time for your intended to also review his financial affairs and make provision for you/your ds accordingly. You may wish to post on the Legal board where mumblechum (hope I've got the name right) is a highly recommended Willwriter and other responders can advise on specific issues.