Bunglie - that's FANTASTIC news! Please, strike while the iron is hot and arrange to meet him ASAP. Don't delay. I think that's particularly important because I REALLY don't trust your stepmonster. She sounds as if she would say or do anything to spoil things for you and keep in with your children. Don't give her the chance to poison the well with lies or innuendo - if she has contact with your kids she could say or do anything behind your back. I don't mean to make you paranoid,but just look at her track record. How DARE she get angry with YOU?? That's amazing. She ruined those kid's lives and your lives and lied and caused your children to be taken away and she's angry with YOU? Is she mad?
Re your son, I would write back immediately, enclosing the photograph and suggest that you might meet for coffee soon as you will be in the area (make up a reason if you want) and thought it might be a good idea to meet up.
I think you should take the first letter with you, in case he wants to know what really happened and you can give it to him.
He wants to see you - and that's just unbelievably wonderful.
BTW don't worry about the texting - he's 18 and all 18 year olds text all day longer and wouldn't dream of writing a letter! it just means he's normal.