Hi Paul - firstly, please do NOT restrict his water right now at all (not that you are
). If he has Diabetes Insipidus (vs Diabetes Mellitus), restricting water could result in death.
The vet can run bloodwork that will rule in/out DM first. Of course, a full blood panel should really be done due to the polydipsia (increased drinking)...bc other things could be going on. He should also be considered for Cushings Disease.
If he does have DiabMell - he'd need to have his blood sugar checked daily, and then given insulin injections (daily, at least). Insipidus, which is more rare - is managed in by other meds that help manage his body fluids.
The nuclear sclerosis can be seen in mid-age / older dogs. Just make sure you're confident that the Vet is sure it's not a cataract....bc cataracts are common w/ DM, and if it is a cataract, it has to be evaluated in order to be treated.
Please keep us posted on this fella
!