If the President questions his service, this will move from a question of the Truth to politics. It will blunt the effectiveness of the non partison mission these veterans have. It will be deemed political by the reality he is the President.
Maybe circumstances will arise down the road that make that questioning warranted from the President, but from my vantage point at this time it serves him best to stay out of this.
They can question his actions after the war. They can questions his Senate record. The Vets are the most qualified to speak about His service in 'Nam.
"Look, there are differences of opinion on the Senator's service record. A lot of what he did in combat and his motivations are known to Senator Kerry alone, and I'm not going to venture to criticize that."
Then follow up with
"On the other hand, what he said and did after he returned from Vietnam - blackening the names of our military at every level - is incontrovertible."