Very difficult.
You need a way to try and resolve this that doesn't immediately escalate it to an official complaint or make the situation worse, or put yourself up as a "troublemaker".
Any new proposal requires a sequence of actions that includes "consultation". You float the idea and solicit comment. You must expect some will be adverse. Adverse comments are always valuable because they need to be dealt with by providing a solution in the proposal not as part of a fire fighting exercise after the fact.
So if you say "we are all going to jump off a high building" it is absolutely no good if everyone says what a good idea it is. You need someone to object and say that you'll all be killed (not find it out on the way down). This problem requires a solution and you then can propose abseiling or parachutes, for example.
So what the objector needs to do is address these comments to you, the proposer, not to the operators. He does need to be asked for feedback and may welcome the opportunity. If no feedback mechanism has been provided some people may find a propaganda campaign the only option.
Ergo, you need to request everyone concerned should submit and response to the proposal and specifically request comments on any adverse aspects. You can send this to everyone (not to single anyone out).
Making his comments to you ought to be the proper response so that his objections can be seriously considered. You never know, asking him directly for his comments may make him feel included rather than an observer and may help bring him onside. It is especially important to deal with objections in a positive way and to show appreciation to those who make the objections because they can be a very valuable part of helping everyone "buy in" to the scheme. Sometimes the very people who raise objections are the ones with the solutions.
You might mention your proposal to formally request comments to your own manager and advise that you have heard there have been some objections and you need to have them formally submitted for consideration.
You don't need to volunteer what you've heard about one person, but if your manager asks (as he ought if you are suitably vague)then you can provide all the evidence as being asked for not volunteered.
Having come up with a course of action designed to solicit and consider any and all negative views, it may now put this engineer on the spot. He can either not submit a report or he can submit one that is totally negative and it is then he who will be demonstrating his negativity.
JMW