@ladyazymith -
You can't "let" your fiance file HOH; he has to meet the qualifications. You have to meet the qualifications also; even for you it is not an "automatic".
there are two rules to file HOH
1) you must provide for at least 50% of the expenses of your household. It isn't clear whether you and your fiance are living together, and if so, only one of you could provide more than 50%.
2) Is there a qualifying dependent that lived with you for at least 6 months? I presume both children live with you, so you should be able to meet this test easily. But it is not clear whether your finance could meet this test (does he live with the children?). Further, if child #1 is not his, then he could only claim this child as an 'other dependent' if the child lived with him for all 12 months of the year. The problem is that until you two are married, there is no legal or biological connection between the fiance and child #1
if you can answer 'yes' to both tests, you can claim HOH.(which means be won't be able to answer yes to both)
if your fiance can answer 'yes' to both tests, he can claim HOH (which means you won'tbe able to answer yes to both) and claim child #1 as an 'other dependent', assumming the additonal requirement of living with the child for all 12 months of the year is met. Child #2 would be his qualifying dependent.
it is possible that neither of you can answer 'yes' to both tests, in which case neither of you can claim HOH. You both would file single (and presumably, you would claim the children or each of you would claim one).
this is complicated and best to use the official IRS calculators. Do the dependent calculator first and then do the Filing Status calculator. Should take 15 minutes at most
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
https://www.irs.gov/help/ita/what-is-my-filing-status
