The letter should be from the landlord themselves because it's the owner of the property who has to give permission and an estate agent doesn't have any claim over the ownership of the property.
Can he not contact his landlord directly to ask for a letter?
The UKVI website states:
(https://www.gov.uk/government/publications/maintenance-and-accommodation-maa/maintenance-and-accommodation-maa--2#maa9-assessing-adequate-accommodation)
Otherwise, the letter looks fine - it should be addressed to The Entry Clearance Officer, so no need for both the 'FAO ENTRY CLEARANCE OFFICER' and the 'To Whom It May Concern'. They should just start the letter: 'To The Entry Clearance Officer'
Ksand-- how does this sound?
Landlord name
(Landlord address goes here)
(Landlord phone number goes here)
30/07/15
To The Entry Clearance Officer,
I am the landlord/owner of (husbands rental home). This property has 4 bedrooms, two bathrooms, and a garage. The property has been previously let for a number of years and is classed as a long term let property.
I can confirm that (my husband) met our tenant application criteria and has resided at this property since 13th June 2015. He is currently within the assured shorthold period however upon the expiry of the assured period (my husband) may be invited to sign a new tenancy agreement alternatively by agreement with both parties the tenancy may become periodic (rolling month-to-month).
I, (landlord) was aware prior to (my husband) taking the tenancy, that his wife, (me) would be moving into the property upon her arrival in the UK- I am happy for this to be the case and am giving my permission .
If you would like to discuss the matter further please call me on (landlord number here)
Yours sincerely
(Landlords signature )– Landlord/homeowner