Gabyto I'm seconding dandi's advice...
Caveat: My thoughts here are of a commercial photographer and may not be in harmony with your needs.
If your son want full frame and has to pay for it himself then the iconic D700 win's hands down in my humble estimation. I shoot the D700 more now than my D3x and D810... it is truly an epic piece of kit. Many wedding shooters still pay the bills with this solidly build professional Nikon FX body.
But please don't take dandi and my word for this...
https://www.youtube.com/watch?v=Cc3cAr5HkF8https://www.youtube.com/watch?v=FpcWSRCprSQhttps://www.youtube.com/watch?v=s4_0iS1oQQchttps://www.youtube.com/watch?v=3xXZVXunMjYAs for long glass?
I shoot portraiture, lifestyle/fashion editorial and sports...
In my experience the vintage AF 180mm f/2.8D IF ED works superbly with all...
Personally I would not encourage anyone to specialize in wildlife in as there is virtually no revenue potential there. Especially BIF which is primarily a hobbyist pursuit for those with deep pockets and again nearly devoid of revenue potential.
Commercial Photography with a D700 and an AF180mm f/2.8 IF ED is indeed a viable option.
Add an AF 85mm f/1.8D Nikkor to this mix and you have an epic event shooter's kit in my humble estimation
KEH, B&H, Robert's are vendors I have had excellent result with for over a decade...
They have stellar used kit warranties and fair return policies..
D700's in good condition $350; an AF 180mm f/2.8 in good condition $400
Once covid19 is eradicated sports is a strong option for your son, I have a major client for league soccer...
Yes you can shoot soccer with an AF 180mm f/2.8...
It is also good for "American Football" and a vast number of "Field" sports.
Gabyto wishing your son all the best on his journey...
Below is an example of the iconic AF 180mm f/2.8 IF ED capturing the starter's gun at a girl's 5K event.
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