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Having read your update it seems your son if feeling insecure about there being a new man around the house, he feels his position with you is threatened. Maybe your son is thinking that if his Dad has found a new family and step children that the same may happen to you and he wont feel a valuable part of the family.
Make sure that you still have time with your son just the two of you as well as with your new partner, and most importantly talk to him about what he is feeling, explain the importance of him as your son and how no-one will ever replace him.
Also explain the positives of having another male around the house, playing football, computer games together, holidays, days out etc.
With regard to his behavior I think this may be a strong willed 8 year old standing his ground, and you should continue to discipline as normal and do not allow him to manipulate you using the situation of the new partner which you know he is sensitive about. I hope this helps you. Please click accept if you are happy with my answer.