Baby sleep can be so frustrating! Some babies just take longer to sleep through the night. It's actually biologically normal for them to not sstn at that age. My DD was like that. Really it was forever before she ever slept for 5 straight hours! Breastfed babies nurse through the night for a long time. We coslept which made it much easier. Your sleep patterns do sync as someone else said and thus when he is in lighter sleep and then wakes, you are also in the lighter sleep so it is SO MUCH less jarring to you. Many women mostly sleep through the night nursing. I didn't but it was much better than having her sleep separately.
It's a myth that introducing solids makes babies sleep through the night. Sometimes solids can make the problem worse, so be sure that what you are giving him, especially at night, is easily digestible. Cereal is just empty calories with added iron, so I don't know how helpful that is at all. We never fed that at all. Have you tried trying to give lots of nursing later in the day/evening? Sometimes that can help. Adding more solids may just make him hungrier in the middle of the night since they are likely to be less nutrition than what is in the breastmilk.
Their sleep schedules make them more wakeful at 20-30 mins, though plenty of them don't actually wake. DD did often wake 20 or 45 mins into a nap. My solution was to try to be right there and get her back down quickly or to have her in a sling for naps (or let her nap on my lap and get her right back down). You can do a lot of stuff with a sleeping baby in a sling-really! Also many babies sleep a lot better on their tummies-that was my DD. And do you use white noise?
As someone else mentioned food intolerances can cause sleep disruption in babies. But what you describe at that young age, sounds like normal baby sleep. They need a lot of milk to support all that growth and development that is going on.
I know it's tough, especially if you are talking to friends who have babies who just sleep well. DD is a great sleeper now at 3. She did outgrow the need to nurse all night eventually. Really! Good luck.