My partner supposedly fathered a child when he was 17 hes now 32 and has not been completely honest with me!?

I need desperate advice form anyone reading this.... have been dating my now fiance for the last 2 years. Apx a year into our relationship he sat me down and told me that he had heard through the gvine that there was a possibility of him fathering a child when he was 17. He made out that he had never seen the child and had no knowledge of this being true.So i belived him when he said he had 0 children.2 1/ 2 years down the road and we are living together and have a now 5 mth old baby.We recently moved home & had mail redirected. Last week i opened the mail to find a letter addressed to my partner from the childs support agency stating that he owed a woman 7,000 in unpaid child support payments. The letter was threatening bailiff action, imprisonment etc if he did not pay up.I have confronted my partner about the letter. he states that he will deal with it in his own time! 0 explanation/ los of tears. It turns out he has seen the child 1 b4 found this out from his sis! i'm angry