How to deal w/autistic child at VBS who is being physical with my son

We are on day 3 of VBS (vacation bible school), my 4YO son is attending. I have been attending as well, and have been staying hands off, to the side, until I noticed a known autistic child who is several years older than my son, being physical with my son (and any other child who is in his direct path). He elbows, pushes, grabs arms, yells in the face, etc. His parents are hardly ever close by, as they are helping with other areas of the VBS. The autistic boy is clearly having a hard time this week with the differences in his daily routine. I'm not sure if I should talk to him about what he's doing when he's doing it. What words would work with him? We have always taught my son that if he is being bullied or hit, to stick up for himself but now that this is actually happening to him, he's been cowering. I get it that he's 4 and putting advice into motion has got to be hard for him. I decided that I need to talk to him before we get to VBS tonight to talk about this kid and his autism at a very low level so he understands, but outside of that, I'm not sure what to do without getting too involved or upsetting parents. We have 2 days left of this. via /r/Parenting http://ift.tt/2w0OfxR