Is he going to be having regular three monthly IV's?
I think, from your terminology, that your DS has the same condition as one of my DC.
I am surprised they have recommended a central line - it's not common to have a central line in my DC's case, but portacaths are extremely common and are placed under the skin. Central lines tend to be for patients who will be in hospital care for a while (or at least from my experience).
Infection rate is low due to being placed under the skin, but obviously they will advise on infection risk when a gripper is in situ, as when it's accessed the infection risk increases.
One of my DC has had a central (when they were a long term inpatient) and has had two portacaths - They are ideal for regular IV use. If your son is going to be having regular IV's then cannulas are best avoided as it puts unnecessary pressure on the veins - they become more difficult to access when having regular antibiotics.
I would really recommend a portacath in situations like ours - I have advised other parents both the reasons for and against but mainly advocate that I agree that they are so much more convenient.
Also your son may be compliant with cannulas at the moment but this may change if his veins become more difficult and placing cannulas is a more traumatic experience.