my daughter is 8 months as well and she doesnt eat solids. contrary to what people believe, breastmilk alone can sustain a baby just fine. the ONLY worry would be iron deficiency since iron stores in a baby start to decline after 6-7 months, however the chances of a healthy, full term breastfed baby being anemic at 9 months is VERY, VERY low. BUT, there is always a but, the research i read says once you start to give a baby iron fortified cereal and formula(which is only absorbed at 4%) and iron supplements, the babies body changes the way it is absorbed and cant absorb much from the breastmilk(which is a very small amount however is absorbed at 50%) so what you should do is have a very minimal test done, and if it comes back low, just increase iron with food(like broccoli). there is no other problem from only eating breastmilk.
now if you are only breastfeeding though, your son will wake up as breastmilk is absorbed so quickly, and at his age and weight, he probaly needs more milk than he can take in all day, so he wakes at night. my daughter wakes up about the same at night. she is a snacker, so eats about evert 2 ours for 5 mins. for solids, have you considered baby led weaning. at 8-9 months, its the perfect age to introduce whole fresh foods. you dont have to feed ANY baby food, you can just skip it all and feed regular, healthy foods. my daughter just had avocado for the first time last night, my next will be a broccoli head. i plan on never having her eat baby food, and eat at her pace. just google it, or go to mothering.com or la leches website in the feeding solids section. my daughter now wakes up about the same times as your son. she is a snacker so eats for 5 mins every 2 hours.
for the naps, i do believe sleep begets sleep but at his age i would cut out the evening nap, and then have him go to bed earlier, around 7 or 7:30. its great that he is such a napper, but if you can get him to instead sleep earlier, it will end up being a better schedule for you and him.
so all this food/nap stuff aside, just know some babies wake every few hours. some young children still wake. every child is different, and if there was some magic pill of what to do to make a baby sleep thru the night, we would all know about it. what works for one, wont work for another. my children are light sleepers. my sisters son slept all night, and took 2 3 hour naps a day. her daughter wakes all the time at night, and never naps. everyone is different, just like adults, so dont get too caught up in following this advice and that, since it may not work with your son. good luck!!!