It's kind of a legal grey area and certainly doesn't look good if something like a buyout happens shortly after. So even if it's not illegal, it carries some reputational risk.
Insider trading plans have nothing to do with 'the law'. There is no such thing as legal or illegal when insiders trade their stock.
The only value of following a plan is that it offers the insider a defense if they get sued for insider trading. If a insider goes against the plan and doesn't sell a previous amount basically no one knows it except the BOD of that company.
On another note, he's 70 years old and might be doing some prudent asset reallocation, plus he still owns 3 million shares.
I wouldn't discount this. He may have faith in the company, but his family could be tired of it. Still, many 70-year-olds are in the prime of their old age.