Ahh, poor little lad. If he is feeling anxious about stuff, it's likely that these are nightmares, as opposed to night terrors. It's pretty hard to tell the difference if he can't explain what the problem is, but nightmares originate from REM sleep and are basically vivid and frightening dreams. They can happen at pretty much any time of the night during REM sleep. Night terrors originate from deep sleep, they come without warning and are not necessarily to do with any sort of anxiety (although anxiety or change in routine etc can make them worse). The sufferer wakes up 10-45 minutes after going to sleep with a feeling of abject terror, and often will not know why. They aren't caused by dreams, but sometimes the sufferer will have some kind of hallucination of something terrifying - for me it's usually some horrible thing in my bedroom or hanging from the ceiling above me. Quite a lot of people sit bolt upright or leap out of bed, they sweat and breathe heavily, and their hearts pound really quickly. You would have to watch your DS to see what he does. If he wimpers and squirms and seems disturbed, and then starts crying for you, it's likely to be a nightmare, you can wake him up, give him a cuddle and reassure him that he's safe. If he seems to startle or suddenly sits up with his eyes wide open screaming, it's a night terror. His eyes may be open but he's probably not awake. You can cuddle him (phsyical contact helps) and repeat something reassuring over and over ("mummy's here, mummy's here, mummy's here...") until his breathing starts to calm down, his heart stops hammering and he starts to cry normally rather than screaming his head off. Then you can go into reassuring mummy mode. If the poor little chap is having night terrors, he may still feel anxious the next day (the hangover phase), and then worry about going to bed at night in case it happens again.
There's not much you can do, tbh, other than understand what's going on. You'll probably find it comes and goes in phases, it does with me. If it's nightmares, I'd keep him in his own bed. A night terror takes some getting over, so he may well benefit from a cuddle in your bed afterwards. It's a tough one to call though, like you say.
I've had both nightmares and night terrors from the age of about 3. It was always worse when something in my life changed (new house, new school, leaving home, changing the furniture round in my room...). These days it's quite rare for me to get them, and now I understand what's happening I can usually get myself out of them. It's a whole lot harder to deal with if you're only 3. Bless him.