First I must tell you that it is never a sure bet when you are talking about a TPR. You did not say what the father said in the email or how long it has been since he sent it. If he asked about the children in any way that counts as an attempt at contact. It must be at least 6 months from the last contact or attempted contact. A TPR is a 2 part test. Even if you meet the grounds for termination, in this case abandonment, then the judge will look at the matter to see if it would be in he children's best interest for the termination to take place. It is a high standard, higher than the standard in a custody case. I have done several TPR's and have seen it go both ways. There is so much that you need to talk to an experienced attorney about before proceeding with this matter. It may very well be the best thing for these children in these circumstances especially your son that is autistic. If you died the father would be the next in line to assume custody and it sounds like that could be devastating for these children. You will need an attorney to proceed on this matter.
I believe you have grounds for termination of parental rights. One email in six years is not a lot of contact.
My firm is experienced in these types of cases. An attorney can help you get through the process effectively and understand the ramifications that termination has on the children, including inheritance rights.