Recommendation letter from a PhD-to-be?

I am applying for Master programs in Educational and Clinical Psychology. And I am in the process of choosing people for writing my recommendation letters. I have a BA in Psychology and I wasn't much involved in research - except when I did my year aboard, I participated in a research project launched by a teaching assistant who was then a PhD student at that college. We conducted the experiments together, collected and analyzed the results. In the end, he supervised my Research Project Thesis.

It was a valuable hands-on research experience for me. And I am thinking about asking him to write my recommendation letter. However, though he is still actively involved in several research, he hasn't yet obtained his PhD degree (he will in the coming year). So I am concerned that a recommendation letter written by him would look nice, but at the same time it would lack some sort of legitimacy and affect my chance of being shortlisted for further interviews.

So my question is - would a recommendation letter written by a non-PhD referee carry less weight and even not taken seriously? Should I ask this person to write the recommendation letter for me?

Thanks for viewing my question and I look forward to your answers. Thank you for your help!