That's not bullying. It's critical, sure. It's not worded the nicest way, but it's not bullying or even picking on him. It's pointing out that your son is not the tidiest eater and the other child is noticing. By ten years old, most children are aware of the etiquette of eating around other people ie. nice and neat and tidy and if they're not, they're responsible for cleaning up after themselves.
Obviously, our children that have Autism can often be a little delayed in these niceties of dining. Sometimes it's issues with personal hygiene. Other children noticing and pointing it out, seemingly from your description not in a mean way just pointing it out, is not bullying.
What I would do is help your son be more aware of his own personal space and that others notice it, and help him with his eating habits and manners. The thing here is that he doesn't notice his own manners and the visual effect.
Food habits are so difficult for our amazing Autists. It's not about changing your son, just helping him navigate and reinforcing basic table manners... and not teaching him that what the boys are doing is "bullying" him or that he needs to defend himself or that you need to interfere or defend him either.
Explain to him that there is etiquette when you eat even when you're alone. There is etiquette when you eat with your family. There is more formal etiquette around other people in public. Practice at home together and include making sure he cleans up after himself. Explain that people his age don't always know how to say things politely or to keep words in their heads. It's not excusing the boys' impolite behavior, but explaining it.
Please also know.... I have been there and done that, wearing the t-shirt and still go through it. My beautiful little autist needs reminders about cleaning up after herself with these basic hygiene issues that we've been reinforcing since toddler-hood and she's almost 11. She's had hurt feelings from other children pointing out a bad manner or two, but with guidance she's learning. But she gets easily hurt feelings because she doesn't always know how to interpret other people's words and behaviors.
Does my heart break for her when things like this happen? Yes. It's so hard to teach her self-awareness. We have Her Bubble and Other People's Bubbles. We talk about how Other People will notice Her Bubble, and she needs to learn to notice Other People's Bubbles and learn why other people say and do certain things. As we do that, she learns to self-modify by choice.
In our case, we have never, ever kept it from her that she has Classic Autism. She knows the reason she has some differences, but she's known long enough that she's proud to be an autist and perfect the way she is and that we don't ever want her to change... we only want to help her through the tough spots. So she embraces it.
Talking these things through, whether he knows about his Autism or not, is imperative. Helping him work through his feelings and how to problem solve, maybe helping him script responses that are polite and thought out, will help him feel more secure. Helping him tidy up his meals will also help him feel more secure. Practice practice practice... and he'll feel secure and confident and he won't NEED to defend himself.