It sounds like you're on the right track - the fact that you're still breastfeeding every 3 hours means that your breast milk is still his primary nutrition source - that's the biggest thing with starting solids. We started also about 5 months on the advice of our ped. With our son (now 3), we tried to get away from rice cereal pretty quickly, just because it isn't very adventurous (taste-wise) OR nutritious. We switched pretty quickly to a multi-grain cereal and an oatmeal (more protein & fiber than rice), and started offering 1 other "taste" at each meal, usually, a few bites of puree'ed veggies or fruit. We waited a week at first before starting a new food, but our son really didn't have any risk signs or problems with any kind of allergies or digestion issues, so we just went for it. We'd try to do about a tablespoon of a new food, and keep trying that food until the jar ran out (about a day). Again, the idea is for him to keep getting the bulk of his nutrition from breastmilk, but to get him used to the idea of new and different tastes and textures, and to start building the mouth & tongue muscles needed for real food down the line.
About the not sleeping through the night... it was our experience (even now) that we have to re-sleep-train periodically, usually due to some disruption in routine, like travel, or illness, or something that throws our schedule & situation off. When he was 6 months old, our pediatrician told us to wait no less than 20 minutes, unless the cry was really urgent, like painful. Since we did Ferber initially to sleep train (so glad!), we try to have our son cry for about 10 minutes before we go fix the issue, even now, though at 3 yrs, usually the cry is pretty urgent when it happens (once every 2 months at most).
I hope this helps!