My thought are with you all...such a difficult time but in my opinion from a personal complex medical history while having 2 teenage sons....be open and  honest with your family...explain what is happening in an open and frank way..be reassuring but honest..important to give them the facts, not to over complicate what's going on..and again resist saying 'everything will be ok ' 

Obviously a good outcome is what everyone is hoping for  but truth...you don't have firm diagnosis and it's important to keep them informed every step of the way..with as much information as you feel comfortable sharing  with them,  along with respecting how much they 'want' to hear or can absorb..

Such a difficult time but I'm sure they are already aware that something is different in the home...

Sending positive wishes to you all..

I absolutely agree with not telling anyone until you have firm results ,when I had to go for a Colonoscopy and a Hysteroscopy for suspected ovarian cancer I didn't tell anyone other than my husband as he was taking me ,I didn't think it was right to worry anyone until I had to ,turns out it was something not at all sinister which I found out on the day of the tests and I was so glad I hadn't said anything ,they don't know to this day I even went for the tests .

I think at the moment, you could just not mention cancer. The bowel issues at least could be many things and no point in emphasising the most serious possibility. 

I think you could just explain that he's been having some problems with his bowels and is having some tests to find out what could be causing it. That isn't even misleading them, It's more accurate than saying he's being investigated for cancer, because cancer is only one possibility.

The melanoma thing is a bit harder in a way, since the possibilities there really are cancer or fine.