Hi, I'm Magdlyn and I am polyamrous~ and so is my 25 year old daughter.
I see some red flags here. I am thinking your new interest is pretty scared to communicate about his feelings for you openly with his gf. I'm not saying this is because of your young ages; I've had things go wrong with married poly middle aged couples as well, in situations similar to yours. One married poly guy I was involved with also kept putting off telling his wife about his feelings for me.
My daughter was trying to practice polyamory, but recently told me she's taking a break from it, because she doesn't feel she knows herself well enough to manage multiple partners... and her young lovers weren't doing too well at communicating or handling jealousy or the many other factors involved either.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve.
~Percy Byshe Shelley