You can try increasing the amount of solids he eats in the day in hope that he will stop waking up at night
You can start porridge any time. But do remember to observe the 4-day rule.
Right now, you can either introduce a new food (eg, vege, fruit) during his lunch (so that any allergic reaction can be picked up during the day), or you can start him on porridge.
One at a time. That's the golden rule.