Very low B.P. and faster heart rate

My husband started on 10 mgm of Lisinopril about 1 month ago after being on hydrochlorothiazide for 4 months. Dr. added it because his B.P. was still elevated and pulse remained rapid (many times over 100). His B.P. is now in the 90's systolic or lower in the morning and after exercise like mowing the lawn, his B.P. went to 78/45 and pulse of 100 after resting for awhile. Feels fatigued. No complaints of any pain. Worried about the low B.P. readings he has been getting even before he takes the pill in the morning.