I'm with Jackrose - often with babies, by the time they yawn they are over-tired (like those ads for Gatorade - if you're thirsty you're already dehydrated). There is a really good book which uses gentle methods called "The No-Cry Sleep Solution" by Robyn Pantley. From memory it suggests keeping a diary of your babies sleep and behaviour patterns so you can try to see some common threads in their reactions. Kinda like you would with a newborn - try to watch him and learn what he is trying to communicate. I know with my kids they have all been different, with Rose now I know when she gets glassy eyed she is tired. She rarely yawns if ever. If I let it get past that stage she starts laughing and being really animated, but it is a pretty slippery slope to her being really jittery (to the point where she almost shakes at noise and stimulation).
Poor love you will get some relief soon - if there is one constant with babies it is that they are always changing HTH