If an employee abuses the power given to him, which is steadying the growth of the company. How do you deal/cope with him? Firing him at the moment would not be an option as he is the top trained engineer by the company.
There is likely a lot of background to this question, so it's difficult to answer online with one specific solution.
To begin, "control freak" and "abuse of power" can be quite subjective terms. Objectively, in what ways do you feel he is overstepping his authority? Is this a recent event, or an ongoing concern? Has he been formally spoken to or reprimanded by senior management in the past about this behaviour?
Hopefully your organization has policies in place to address behaviour like this. Setting clear expectations around what is and isn't acceptable, and turning to progressive discipline if necessary, often help the employee to understand the outcomes (possible suspension, demotion, termination etc), should they continue down this path.
It's important however to first understand and deal with the root cause of this behaviour, not just the symptoms. Speak with the individual candidly and honestly, and ask them if they feel it's appropriate to behave like this, and try to reveal WHY it's happening. Perhaps they're having difficulties in their personal life, or they're stressed at work. Be respectful, and work with them as best you can.
If you would like additional help, please arrange a call with me. I would be very happy to help you get through this in a way that is best for the employee, and your company.