Your son has recognized a difficulty with defining everything as solid, liquid, or gas. Congratulations. Yes, there is a broad spectrum of “consistencies” between the most liquid and solid. It is a property known as viscosity. It is measured as the amount of time it takes the material to flow through a given opening, or the pressure needed to force it through.
There is a whole field of specialized science/product manufacturing dealing with how to give things the desired viscosity for a given use. Think of tooth past, skin lotion, engine oil, paint. An added complication is that viscosity of most things changes gradually or markedly with temperature. I left this out of the text to avoid the extra complication at the beginning level, but I am happy to see that your son recognized it.