Are you familiar with the Token Economy method? It’s good for encouraging a person to work towards a specific goal. Matthew’s teachers & school psychologist use this technique.
If he performs the activity (or desired behavior) he gets a sticker on his chart. After Matt has earned 4 stickers he gets to play games on his school Chromebook for a short time. At home people might earn a 1/2 hour of watching TV. The small reward should be given in a minute or so and not taken away. It should not be food or drink. Some therapists (OT, speech) have a prize box full of little cheap fidget toys. Matthew recently brought home a mini plastic slinky.

Strategies to Increase Motivation
Leverage Special Interests: Incorporate highly preferred topics or activities into learning or daily routines to spark intrinsic motivation.
Positive Reinforcement & Rewards: Use tangible rewards, praise, or extra time with preferred items immediately after a positive behavior to strengthen it.
Structure and Visual Aids: Use visual schedules, checklists, or timers to reduce anxiety, help with transitions, and clarify expectations.
Break Down Tasks: Divide complex tasks into smaller, achievable steps to prevent overwhelm and foster a sense of accomplishment.
Offer Choices: Provide opportunities for decision-making (e.g., "Do you want to do math or reading first?") to increase autonomy and motivation.
Supportive Environment: Create a safe, encouraging space that celebrates effort rather than perfection.
Follow-Up with Preferred Activity: Use the "first-then" approach: complete a less-preferred task first, followed by a highly-preferred activity.

Understanding Autistic Motivation
Intrinsic vs. Extrinsic: While intrinsic motivation (internal drive) is ideal, external rewards are often necessary to build initial momentum before transitioning to intrinsic motivation.
Social Motivation Differences: Autistic individuals may have lower, or different, motivation for social interaction compared to neurotypical peers, often preferring to focus on specific, engaging tasks.
Sensory and Emotional Factors: A supportive sensory environment is critical, as sensory overload can severely drain motivation.
Need for Competence: Fostering a sense of competence and pride in their accomplishments is a strong, sustainable, long-term motivator.

https://pmc.ncbi.nlm.nih.gov/articles/PMC2926912/
https://www.autismspeaks.org/tool-kit-excerpt/h...