It's a common misconception that having your own other partner will somehow prevent you from feeling jealous about your partner's other partner. It seems like a lot of people new to polyamory fall into that trap.
It's not hard to see where that thinking comes from. I think fundamentally, it's mistaking "jealousy" for "envy." Envy is like... you have cake, so I want cake. If I get cake, then I will no longer envy your cake. But jealousy is more complex. It usually stems from insecurity. I made you cookies. Oh, that guy just gave you cake. His cake is probably better than my cookies. Now you won't want my cookies. I'm sad because my cookies aren't good enough for you.
At the end of the day, it will be his responsibility to deal with his jealousy. You can be loving and supportive and understanding, but he'll have to do the hard work to feel better about this. There are lots of threads here and resources online for coping, just search for "jealousy" on here or "polyamory jealousy" on Google.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker