Welcome to SR. Glad you have decided to post what is going on in your recovery from an unhealthy relationship. Others can gain strength from your journey and take comfort that they are not alone.
I definitely agree with what has been said in that 28 days is hardly enough time to roll out the welcome wagon on recovery. He has received the tools to maintaining sobriety and now it is up to him to put them into practice. From the sound of it, he has not gotten off on the best foot since he couldn't even tell the truth about the particulars of his leaving rehab.
Could it be that he is just laying the ground work by reaching out to you in case his current situation doesn't work out. Considering he's probably lying to her too.
Whatever the drama he's trying to stir up, ignore it. You have a life to keep living that he is no longer allowed to muck up.
I wish you and your DS the very best!
Alice