I think the person to ask is the one who looked at your X-ray and decided it was probably a bit weak! It is so, that you will reckon with a year before the bone is finished with rebuilding itself. Sensible light weights will help. Did the guy say for how long he wants only 5lbs? Do you have another outpatient appointment when you can go and tell him how you are getting on and ask if you can increase the weights slowly?
You had none- or severely delayed-union of your humerus over 5 months for some reason. The guys who push more weight sooner haven't had that problem! The plates keep the site stable while your bone graft etc is healing. The surgeon seems to want to play safe and not risk a re-fracture of the site that is not yet fully healed. Sensible to follow his advice I would say.
All the best, keep patient!