Since he has 5000iu no sense letting it go to waste, may as well start it a couple weeks earlier, say 7-12. It will give you a nice natural test boost as well so keep that in mind when planning AI dose.
Usually (granted I have not seen them all) the HCG pwder is in a vial large enough to hold enough water to add 1ml water per 1000iu hcg and therefor designed to have the water added directly to the vial holding the hcg powder.
Just make sure you use BAW if you want it to last 5 weeks.