Thank you for your question!
I'm glad that you are considering CHC's materials for next year. One aspect to remember when switching from one program to another, is that while the basic concepts are taught, the order in which they are taught, the way they are introduced, and the practice provided will vary greatly. When you switch to CHC, you might find that your student will be ahead in one area, at level in other areas, or not familiar at all with new concepts.
I'm very happy that your four year old is doing well in school. This is great, because he is excelling and loving school. This is helping build that foundation and the love for learning that he will need as he progresses. It is important to ensure that he doesn't become frustrated or tired of school, because this will affect him as he progresses through the older grades.
One problem with enrolling him in 2nd grade, is that he obviously will not be ready for the First Holy Communion preparation. Also, you have to remember that there will be even more writing expected from your son in the 2nd grade curriculum. At four years old, he is still developing his fine motor skills, and you might find that he'll need more time and practice with the handwriting level 1 before he is ready for 2nd grade level writing, which could be extremely frustrating at his level of development. In fact, at this age, formal handwriting instruction beyond simple letter formation might be frustrating for him.
My suggestion is that you spend a year improving his reading and math skills. It's so important to lay a strong foundation for him. If you see he is having trouble with particular concepts, or hesitates in certain areas, this is where you can really provide extra practice and review. In reading, you can expose him to so many books that will stretch his mind and reinforce phonics, and simply let him revel in discovering the world that books open to him. I would suggest buying the kindergarten and/or 1st grade lesson plans, and use them as enrichment, to ensure that he knows everything taught in those grades, yet without the pressure of formal school.
At age 4, it's also so important to let your child grow not only academically, but also socially, physically and emotionally. This is such a fun age, because he is learning the why, and how, in the world around him. You can teach so much to him, outside the classroom, that will encourage his growth as a person and child of God.
God bless you and your family!