DS is 16.5 weeks.
Was breastfed for 14, 2 hourly ish - always woke 2 and 5am for feeds
Switched to formula at week 14 for a week and pattern continued
Switched to hungry baby on advice of HV and pharmacist (he is 18lbs!! born at 7.5 lbs so large weight gain) and he still wakes but now it can be any time, always at least twice.
He feeds at one of the wakings but this week we have tried to offer water, dummy, resettling at the other wakings and its going horribly wrong .....
Last night he woke at 330am wide awake, happy, not hungry (having drank 7oz at 1am) and just wanted to play ... until 530am
We have toddler who then woke at 630am so this is not something which is do-able long term 
His days roughly go like this.
7am feed
9am sleep for 30 mins
10am feed
1145 sleep (unfortunatey he usually wakes after 30 mins, sometimes I can persuade him to go back but others he is just awake)
Between 1145 and 2pm regardless its "quiet time" which unfortunately means a lot of dummy/walking right now.
230pm feed
awake, happy
4pm sleep for 30 mins
515pm feed, bath, feed
6pm ready for bed
10pm dream feed
nights - anything ......
Any tips???