Dear Ivyie,
If you have all the documents that back him up that he has been trying to be a legal resident in the u.k, you can attach them but this does not guarantee anything really as you should have mentioned it to them originally that he is an illegal resident in the u.k.
They might consider your case if he has tried to be dependant but was refused in the u.k but remember, if they are the one to discover that he is an illegal reisdent in u.k and there has been an error due to a concealment of a material fact by an applicant, they don't take it lightly. Write them a letter to explain the whole situation, the reason(s) why you didn't inform them before, etc. They will contact you the next thing you need to do, whether you should come for interview with your husband or whether to go ahead and submit your passports or whether they will refuse you. I am not an immigration officer, I can't tell you exactly what they will say but by so doing, you will be able to sleep with all your eyes closed. Remember the truth shall alsways prevail so even if you are refused this time around, you can re-apply without concealing any information from them.