Really, as long as he is still gaining weight and finding himself some sort of centile curve, you really don't need to worry about not having enough milk.
If it's any comfort, between 8 and 12w, my DD gained 180g - that's an average of 45g per week, or 90g in a fortnight. My health visitor wanted to keep an eye on it as the weight gain wasn't massive, but at the same time wasn't panicking as weight was going on, albeit slowly.
For now, I would probably continue to offer both sides at each feed. It is perfectly normal at around 4m for your breasts to not feel as "full" (or engorged) as your body is now producing milk in response to demand from your baby rather than high levels of milk producing hormones.
Is he crying at each feed or just some? It could be that he's uncomfortable or that you have now hit the 4m growth spurt and he's trying to get as much milk as possible, so you may be needing to feed a little more frequently for him and so your body can catch up with his requirements.