Baby prefer formula over BM

I have been giving my 5 month old EBM during the day and formula during the night. In recent weeks, LO has been drinking less than usual. He used to drink about 830ml per day. Now drop to about 700ml plus, sometimes even 600ml plus. On a few occasions, we fed EBM/freshly expressed BM until LO got fussy and switched to formula. He continued to drink more after the switch. My husband is convinced that LO prefers formula over EBM. Is it possible?