Helping a teenage boy with autism manage personal activities like masturbation in a safe, private, and hygienic way requires understanding, respect, and practical strategies. Here's a step-by-step approach tailored for a 15-year-old who cannot be left unsupervised due to risk of injury but needs privacy and to minimize mess:

1. Establish a Comfortable and Private Space:
- Arrange a room or area that feels safe and comfortable for him. This could be his bedroom.
- Use privacy aids like a partially closed door, screen, or a sign to indicate when he needs time alone, if possible.

2. Supervised Privacy:
- Since he cannot be fully unsupervised, consider staying nearby but out of direct sight (e.g., sitting just outside the door or in an adjacent room). This balance offers safety while respecting his need for privacy.

3. Hygiene Preparation:
- Provide materials like tissues, wipes, or a towel to help contain and clean up.
- Teach or remind him to use these materials before, during, and after masturbation to maintain cleanliness.
- You can demonstrate or role-play hygienic practices if appropriate.

4. Protective Measures to Prevent Mess:
- Suggest using a washable or disposable tray, towel, or absorbent pad placed on the bed or floor to catch any fluids.
- Keep cleaning supplies readily available nearby to make cleanup easy and non-stressful.

5. Communication and Consent:
- Talk openly (in an age-appropriate way) about privacy, boundaries, and safety.
- Encourage him to let you know when he needs assistance or when he's done.
- Respect his feelings and try to remain calm and nonjudgmental.

6. Consider Professional Guidance:
- If the risk of injury is high, or behaviors are challenging, consult healthcare providers like occupational therapists, behavioral therapists, or sexuality educators who specialize in autism.

By combining respectful supervision, preparation, and communication, you can help a teen boy with autism manage masturbation safely and hygienically while preserving dignity and comfort.