I am ever so sorry; watching a piggy is always totally heart-breaking! The first time is the worst because you are not braced for just how physical the process often is. When we think of death, we have this vague image of peacefull slipping away in one's sleep. This actually happens only rarely.
Please be strong for your boy and keep in mind that at some point he will be no longer fully or at all with it long before the body goes. It will hopefully not last all that long, especially when the heart is going first. Please be aware that oxygen starvation from a failing heart may cause fitting; we call this 'running to the Rainbow Bridge' as your boy will by then be nearly there. It can be very upsetting if you don't know this.
Watching it can be worse than going through it yourself because you feel so helpless. Once he has crossed the line of no return, there is nothing you can do for him except to keep him nicely warm but not hot (to counter a failing blood circulation which means that he will feel cold) and as calm as possible where he feels at home while he is still with it.
Take good care of yourself, too, and let your boy make his journey carried by the wings of your love!
I am going to bed now as I have a very busy week ahead.
You may find these guides here very helpful if you lose your boy:
Looking After A Bereaved Guinea Pig
Human Bereavement - Grieving, coping tips and support links for guinea pig owners and their children