My second son is 3.5 months old. From around 6 weeks he was fairly good at sleeping, only waking 2-3 times a night for a feed (breastfed). For the last 10 days or so, things have gone sharply downhill; first he started waking every 2 hours, then every 90 minutes. When he wakes I feed him and put him back in his cot - he usually goes down without too much fuss in the first half of the night but gets increasingly difficult after around 2am and then insists on getting up and starting the day between 5.30 and 6.30am. Last night he went to bed at the normal time of 7pm, woke up every hour until 2am and then cried solidly until 5.30am. We're at the end of our tether.
Does anyone have any idea what's going on, and what we can do to fix this?