Now as I am sure you can appreciate, the prognosis for any cancer type will be based on how aggressive the tumor is and where it is located. Unfortunately, your lad has a less then ideally placed adenocarcinoma and everything you have described (the straining, the feeling that he has to go because of the mass's presence in his colon, the compressed stool that is having to slide past mass, the blood, and groaning) suggests that it is already causing him a degree of struggle.
In regards ***** ***** we cannot truly know how fast an individual cancer in an individual is going to grow (not even for people). Generally speaking, the reports on this type of mass without any treatment give a prognosis from anywhere between 6-15 months. That said, if it is already large enough to induce the clinical signs you have reported, we are likely already well into that time frame.
Therefore, with all of this in mind, as long as his struggles doesn't appear to be rapidly progressing, it is likely safe that you will be able to go on holiday. That said, if he is struggling as much as you have described, it may be worth ringing his vet to discuss the use of stool softeners or GI lubricants (ie Lactulose, liquid paraffin, etc) to ease his passage of stool. That could just take some of the strain off of him when he is passing his motions. Furthermore, it would be worth discussing the use of steroids or at least a dog safe anti-inflammatory (ie Metacam, Rimadyl, etc). The reason is because both will reduce swelling and inflammation associated with the mass. That could reduce its size to a degree and again help him with his motions and delay it reaching a critical size. As well, the anti-inflamatories could soothe that inflamed GI and reduce the straining and frequency of the blood you have seen. Finally, steroids are known to have some cytotoxic effects and can be useful against some cancers. So, these would be palliative measures to consider for keeping him comfortable and giving him as much good quality time as we can.
Finally, I do want to direct you to a pain scale (HERE). It is used for assessing discomfort post surgery but the concept works for any condition. It is just a means for you to appreciate his non-verbal clues about how uncomfortable he may be with this. This will just help you to keep an eye out for any signs that he may be starting to suffer and that it is time.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,