Liz,
I would suggest taking his blood pressure periodically, a few times a day at same times, but also when he feels dizzy. Write them in a list (or a spreadsheet, or whatever), and take this with you to the GP. A set of home readings is much more useful to the GP than one taken by the GP in the surgery.
Low blood pressure is not the only reason for light headedness of course. Another possible side effect of some (possibly all) HT drugs is lengthening the QT part of the ECG which gives an erratic pulse rate - that would need an ECG test which your surgery might be able to do. You could check pulse rate and see if it's regular during a dizzy spell (and at other times too).
I haven't had any dizzyness, but Zoladex seems to have pushed my blood pressure up.