Trust your instinct po, parang its an opening na po yan na magka-balikan sila. If talaga po na mahal ka niya try to reach out and talk about it. Remember po na you are entitled to tell him that you are uncomfortable in a relationship with someone who communicates with recent ex for no good reason.
And if he argues about it, ibreak mo na lang po siya. Parang nag-aantay nalang sila kung ganon na mag breakdown ka and napaka-toxic po niyan wag pong martyr.
Kung pumalag pa siya then save the energy and time. Find someone who respects and values you more than anyone else.