Risk of dual submission

I am a USA resident physician hoping to publish an article soon, in time for fellowship applications. Another doctor taking care of the same patient also wanted to submit a case report to a different journal, first as a poster (at a conference several months from now) then as a publication. He was nice enough to reach out, asking if I had similar plans to publish, and whether I wanted to collaborate/co-author. I responded that I had already written up an article for publication. He asked if I had submitted it to a journal yet, and that both articles could potentially be submitted to different journals, as our write-ups would be very different in content, educational emphasis, and intended audience. I told him I hadn’t submitted my article yet, but that I agreed that both versions would be valuable for different audiences. I sent him a copy of my article, with hopes for his feedback so that I may include him as a co-author. I haven’t heard back for over a week now, and I have no more time to spare if I want to include my submission on my CV (which he is aware of, since I mentioned it in my first response to him). Am I being ignored on purpose? Does he foresee a problem with potential dual submission, and is he trying to submit his own poster+publication while leaving me hanging? I don’t feel comfortable submitting my article without his permission; can I?