Among the things that are often recommended for communicating with Aspies are to be direct, honest, and concise.
"Direct" implies "literal" (i.e. no indirect speech. no assumptions about tone of voice, body language, etc).
You need to consider about what you actually want to do, and why you're doing it.
And then think very hard about what "I love you" will mean when you use it. He won't know.
It's not just 3 words. To Aspies, with large innate gaps in their communication skills, notably in this very area, it's not just the usual minefield. It's a book worth of information plus an extended version of the usual minefield.
I think you have an obligation, and if you genuinely want an LTR a duty, to fully and accurately explain the nature of this important step to him. That means more than 3 words.