According to villains.fandom.com, yes, his head is a sandwich.
Chuck's most distinguished feature compared to the other antagonists is that he has a sandwich for a head, and it appears to be an inherited trait since his brother Brent also has this feature. However, it has not been shown if his mother also shares this trait, since she is never seen.
This is backed up by some other internet sources but they could all be quoting from the same anonymous source.
The closest thing I could find to a canon source comes from nhpbs.org.
In this clip, WordGirl and Huggy are held captive by the Evil Sandwich Making Guy. He has a sandwich for a head and mustard and catsup on his utility belt. He uses hot dogs to tie up WordGirl and Huggy.
source New Hampshire Public Television
Word Girl is a PBS Kids show and NHPBS is a PBS affiliate.