Well... I first experienced dysphoria at age 5 but didn't transition until 35 years later.
I have two daughters: the eldest is stereotypically girly, but the youngest loved playing with Lego & fire engines and dressing in boys' clothes. When she was around the same age as your son, she wanted to dress up as Ben 10, The Hulk, Spider-Man... that sort of thing.
I had no idea whether she might be trans; the only thing I knew is that she preferred boys' clothes and wanted to be Spider-Man. So we bought her Spider-Man costumes, and boys' clothes, and fire engines... basically, we let her express herself however she wanted to. She never asked for a male name or pronouns, though - which became significant later.
Fast-forward a couple of years, and as soon as puberty hit she actually became embarrassed by her previous boyish behaviour. She grew her hair long, started wearing make-up, and became comfortable with the changes in her body. That's something I never experienced; puberty was hell on Earth for me.
So she's (apparently) cis. She wanted to be boyish when she was little, so we let her be boyish. She wants to be feminine now, so we support her in that too. That's a text-book definition of a tomboy. So my advice to you? Right now, it's just a dress. It's only fabric. So let your son express himself however he wishes: it may just be a phase, or it may be something more serious. You won't know until later, but letting him express himself is the best way to find out how serious it may be.
