The symbol I would maybe use for time out would be a positive one, because he need time for himself and not to feel upset... maybe I would choose the following one:

How many people that are with him will understand the hand sign? That is what counts ... Most meltdowns are all about communication... not being able to be understood and to understand. ...I usually tell the new staff that come to work with these children: Imagine if you went to Japan, another language, another culture and people would just talk to you and drag you around to the places... what would it make you feel? what would you really understand? Ye if a Japanese person started giving you some chop sticks and then take you always to eat a bowl of rice with him...you would think Wow...chop sticks mean food nhammmy... the first time he would sway the chopsticks for a picture, remember different culture, different you might get angry... "What happened to my chop sticks" I'm hungry" yet after several using it any everyone understanding you would... end up thinking like a Japanese 🙂 I hope I was understandable in my far from perfect English

Since we have a snack at break time the symbol is a boy having lunch. What does your son usually do at break.time so he may relate to the symbol?

You can find on youtube all makaton sign just google

Yes PECs used or picture which he recognises. But I thought he could progress to signing for it - signing for biscuit or cookie or crisps