Hi!
I am very sorry; getting an incurable diagnosis is always a blow to the guts!
I know how great your desire is to do more to slow down cancer, but there is sadly nothing that we can recommend. Please be aware that different species of animals can react very differently to medications, so any home treatment on spec can do potentially as much harm as it can do good.
The good news is that guinea pigs have a much better tolerance for metacam in terms of higher dosages and long term use when it comes to painkillers once that becomes an issue.
Please try to see every day with your boy as a special gift. When and what your beloved ones die from is out of your control, but how you spend the time you have got is the area where you can really make a difference and where you have still got plenty of time to create special and lasting memories! That is the one boon you have got that a sudden illness or death will deny you.
Your grieving process starts when you become aware that a relationship is on a limited time span; the realisation is one of the lowest points apart from the death itself. But you are so much freer in how you can deal with it than when you have to cope with the shock AND the loss in a sudden death. It is a bittersweet time, but you can pack so much love and happiness into a very small moment that you can get a lot out of it, so that you can take leave with a heavy heart, but no regrets whenever the time comes.
Be aware that a month is roughly the equivalent of a year in a guinea pig life.
I hope that this helps you make the best out of what you and your boy have been dealt with.