Frequent venepuncture in itself can lead to difficulties with obtaining subsequent samples. Usually the superficial veins have 'collapsed' meaning that the blood will not flow into the syringe/needle.
He needs to ensure that his blood volume is normal by drinking adequate fluids/water during the day, and that his blood pressure is monitored.
If a skilled practitioner is unable to find a vein using butterfly cannula and normal techniques then depending on the need for the blood test, a more intrusive cannula can be inserted under radiography guidance. Alternatively the radial artery can be used for small samples by an experienced professional and the appropriate needle.
In practice I find that doctors will often use veins that nurses/phlebotomists won't ( eg hand veins) so admission to general ward should allow blood access.
I do not think that it is a "excessive use" problem. Before going to have blood drawn for lab work last Sept. he drank alot of fluids and they still had trouble. What type of specialist would we see to further investigate this situation?
Unless there was evidence of stiffening of skin/tendons which indicates an underlying connective tissue disorder then this problem of 'difficult venepuncture' is very common in people who have had many blood samples or intravenous drips. We see it very commonly in people who have had long stay in hospital. It does not usually indicate an underlying problem in itself.
If you wished an evaluation then a vascular doctor would be the place to start to look at venous flow with doppler ultrasound, but I have never actually seen this done in practice.