I started using ARASAAC because the signs are legally free, easy to make in coulor and black and white and you can change the subtitles. I also use https://picto4.me/ , also free where tI think hey have Makaton. It is very intuitive and tou can add real pictures and make talking mats :)

The sign for ballon using arasaac is:

Hi Indie before using any sigh I am sure you use the real object, the picture so your son can relate and then a sign to compare to the picture. If you send me a picture of what you wish I can put a free international sigh with the name of it underneath so everyone can read even your son.

You can find makaton on youtube and all signs

I will try tomorrow to get picture or photo up - thanks