Has he perhaps lost one of his friends from last year? Or was he particularly attached to his old carers? Change can be hard at that age.
His new teacher (especially if fairly young) may not yet have the confidence to get into conversation with parents, so you might have to approach her. Also, be aware that carers have supervision as a first priority, so if you try to get into conversation when they are trying to watch a yard or room full of children, you probably won't have their full attention. But you are well within your rights to ask for a special time to discuss any aspect of your DS's care that you want to know more about.
Also, there should be some sort of method for you to add your own suggestions to the program. If you think your DS is showing a particular interest in, say, dinosaurs, you can tell (or write down) that info to his teacher who can use that as an observation in order to include that interest in his individual program.
Like the others have said, he is learning by playing. And there are probably a lot of planned activities that you wouldn't necessarily notice because they might not make use of resources as such - maybe more that the teacher is facilitating social interactions, encouraging speech or confidence levels, etc.
If you really want to know exactly what the carers are working on with him, ask to see his observation records - they can't say no to that. But do expect to have to make an appointment so the teacher can go over things with you.
Hope he is feeling happier soon - have you talked to his teacher about that? Ask the assistant too, she see everything that goes on, and develops relationships with the kids, and may be able to provide insight into why he's not so cheery.