The blood pressure of 185/85 is high (with possibility of high risk complications) in a person of 75 years of age. I would recommend you to repeat the measurement after he is settled in and relaxed after 20-30 minutes.
If he is on Blood pressure medication he should take it. If you he has missed any medication, do take it. Repeat the BP - and if it is still above 180/90 mmHG he needs urgent medical care.
Now whether he should go to ER or not - Well its is bit tricky. If he has (1) easy access to your GP - can get in touch with him at least over phone (2) and he is already on treatment (3) with no additional symptoms - you can visit/phone him.
But (1) if this blood pressure is noted for first time (2) if he has any additional symptoms like chest pain, palpitation, syncope, tiredness (3) No immediate access to GP (4) has past history of heart diseases, stroke - he should not hesitate in visiting the ER.
Hope this helps
Feel free to ask followups