You can look at Sonlight as your curricula and at CC as academic enrichment. It is the one day you can get out of the house and CC enhances what your children are learning, gives your kids timeline and historical “hooks” to put their information on when they learn it. In CC they memorize time line information as well as one sentence summaries of historical events. In Foundations they learn memory facts on different subjects: Bible, history, science, Latin, Math, Grammar and Geography. They also cover art, music and a science experiment along with a weekly presentation in front of their class of peers and a family presentation in front of the whole group once a year.
Essentials covers grammar in-depth, IEW writing and math games (to help them get the facts down pat). There isn’t any reading or in depth study. It is mostly an academically strong enrichment that compliments Sonlight nicely. You would still do a full core but usually you start each day with about a 15 minute review of their CC memory work for that week. The children also make oral presentations in CC which gives them practice with something that isn’t easy to replicate at home.
If you do the afternoon Essentials in CC your son will not need to use the Sonlight Language Arts as it includes grammar and writing.
All this to say Classical Conversation and Sonlight work well together. If the CC cycle of the year matches your Sonlight core theme, that is great, but it is not necessary. If your child is doing Sonlight American History, for example, and the CC cycle is Ancient History, it can still work. Just look at SL as your curricula and the memory work your child is doing in CC will serve him well when he gets to Ancient history. He will have memorized information that will be reinforced when he studies that time period in more depth in later years.