That's awesome. It's so cool that he can express himself through art work.

It's important to remember that five year old alters do not literally possess the skills and abilities of an actual five year old. They do possess an element of developmental stuckness as they were split off and separately contained, but some types of memory and learning cross the amnesiac barriers. For instance narrative and declarative memory might be restricted to an alter, but the ability to walk and talk, feed oneself, or even complex actions such as reading and writing or even driving a car might be shared. Or they might not be. It may depend on what was needed.
Just saying that comparing the skills of an alter to a same aged child doesn't work. An alter may be only say 8 years old, but they could either share or "inherit" the ability to do a certain task from other alters, or they could have been practicing an 8 year old skill for 20 years or more. A child alter really isn't the developmental equivalent of an actual child of the same age.

In terms of art a member of my own system is apparently quite artistic because people sometimes show us paintings or whatever that apparently "I" made for them years ago, but I have absolutely no memory of either creating them or even seeing them before in my lifetime, so I don't know how that works. Even though I know the reason is dissociation it still really astounds me when it happens. I have no concept of myself as an artist and can't draw or paint anything.