UGH 😣😴 plz help!

My LO is almost four months old and USED to sleep 6-7 hours through the night, but now for the last month or so he’s only been sleeping about four hours (most times about 10:30 to 2) then every hour and a half until 8 or 9 am. If I try to shush him back to sleep with his pacifier, it only works for about an hour or two. Are there any tips/tricks to getting your baby to stay asleep through the night??? We’ve tried bedtime routine, noise machine, lullabies...I am desperate. 😩 He used to sleep so good, but he hasn’t slept more than four and a half hours in almost two months. 😭