As noted, you do not have to be incorporated as a sole proprietor. It might offer some tax advantages and some liability protection, but is not a requirement. A sole proprietorship is a valid form of a business entity.
Licensing varies with locale. Most municipalities require an occupational license, without regard to your engineering license. It just licenses you as a business. If you name your business something besides your own name, you will likely have to register under a fictitious name statute. Many state engineering boards do not require a certificate of authorization to practice as a sole proprietor...only your engineering license. Again, if you name the business, they will likely require a certificate of authorization for that business.
Check with your state board and your local taxing authority about the two different forms of licensing.