1) As he is a consultant then no relieving letter is mandatory.
2) Apart from that he just worked 2 months.
If still you are planning to give then you can mention as
Towhomsoever it may concern
This is to certify (Name) was engaged with our organisation for the period of (From Date & To Date) as (Designation)
You can add anything more if required
30th December 2015 From India, Bangalore
At the outset we take, this opportunity to Wish you Happy New Year 2016 from Optimus Consultants.
Further in response to your query, it is advisable to issue a relieving Letter to the Consultant(s) if he(they) is(are) issued with an appointment Letter & is(are) on the Pay Roll of Company.
All such Companies who hire the services of Consultant(s) always complete all the exit formalities.
1st January 2016 From India