K.I.
I know all too well how miserable & frustrating sleep deprivation from a sleepless baby can be!
From my experience, traveling or getting out of routine for a while really can majorly throw off a baby's sleep pattern. That was my first guess as to the cause of his sleep changes.
Secondly, I've read that around 4-5 mos it's common for good sleepers to start fighting sleep. Just stick with a routine & let them cry themselves to sleep when it's bed time & in the middle of the night when they wake b/f it's time to get up.
That's my advice. It's awful hard to do, but it is the ONLY way we helped get our son back on track. Now we'll be dealing with this again soon with our daughter.
I've read all the popular sleep books out there & tried all of the alternatives to crying it out. After the most long & exhausting weeks of trying a lot of those methods out, which involved hours of crying on our son's part(& mine) even though they're supposed to be no-cry solutions, we let him cry himself to sleep & saw such quick results (3 days). We did this when he was 10 mos old, though it all started when he was about 4 mos old. "They" say you can let a baby cry when he's 4-6 mos old. I was just too scared to do it earlier.
I do hope this helps & that you can regain some normalcy & good sleep soon!