My Dear,
I did not recognize as Offer Letter is compulsory under common laws, or any kind of laws in the world, but issuing a letter, may cause you in trouble,
As a senior consultant to my company, wish you issue an offer letter to him now, you may not know below scenario, let say for example:
Different date to issue offer letter and termination date of the employment, will you back date of issuing offer letter to August, that is not good, and it is not necessary, because, as you can see, offer letter, is not compulsory;likewise, wish you date of issuing letter today, that is messy, and having no sense at all, he is terminated his employment, then what is the purpose of the offering letter requested by him?
the answer is, he may use that offer letter, to submit to labour union organization,(or the like)to claim on your company of entering into employment contract, and termination of that one is wrongfully.
That is what I am thinking right now. please do check with your law