My approach - will tell him directly and firmly that I appreciat

I will write clearly in my email too this way, if he does not support the parts I need, I am not the one to be held accountable. 

Years ago, the same company, one person yelled at me for not accepting his help which was not needed and wrong. And anothe woman one year ago offered to help, she made a 30 minutes automation into 3 month long eventually failed projects - she insisted to use a method will never work. 

Not sure why people act like this, but I will say NO this time.