Hi,
My first one was a horrible sleeper, whatever I tried didn't work! So although I'm a 2nd time mom, I really sucked badly the 1st time and not exactly an expert.
Best advice I can give you is to buy the BabySense book called "Sleep Sense" (available at any baby shop). It explains a lot of stuff, I wish I bought it earlier and not when my 1st was already a few months old. It's easy to read and follow. About R150.
In essence, your baby can only be awake for a certain amount of time according to age. At 6 - 12 weeks a baby should be awake for 60 - 80 minutes before going back to sleep again. The trick is to get them to sleep at the end of 80min!
The book explains the signals babies give us when they are getting tired, you need to learn to recognise your baby's signals and put him down to sleep immediately. Like Vendah says, drowsy but not asleep.
What you can do right now (before reading the book), is to play some white noise in the background (I'm using the BabySense "womb to world" cd and seems to be working). Wrap him snugly so that he's arms are under a blanket (Dehan doesn't like to be swaddled, but I still cover him with a blanket). If he likes a dummy, give it to him (mine doesn't like the dummy!). I know you are breastfeeding exclusively, but be careful that you don't fall into the trap of becoming a human dummy. So, he gives you a sign that he's getting tired, feed him (if needed), you put him in his bed and play the background white noise. Stroke him on his nose and brush your thumbs lightly over his eyes, babies respond well to that. They also like being massaged lightly at their temples. You can buy some "cradle cap" cream and lightly massage his head. Cradle cap cream is designed for their heads, not oily like normal massage oil and absorbed well by the scalp.
If he happens to fall asleep before you put him down, when you take him to his bed, make sure you don't put him down head first. Meaning, when you hold his little body to put him down on his bed, make sure his head never dips lower than the rest of this body. His bum and back must always touch the mattress first. Otherwise it disturbs the fluid in his middle ear and he will wake up immediately.
I'm not a fan of letting a baby cry, if you've read the book and tried everything but he still cries, then you pick him up and give him a cuddle. Sometimes it's all they want. It might be frustrating for you but they are little for only a short period of time. That said, I also believe that you should start immediately and not when you go back to work.
And even if you've done all that, learned his "tired" signals, succeed in putting him down and letting him fall asleep by himself...even then you'll have days when it doesn't work. I had a whole week (2 weeks ago) where he barely slept the whole week, and yesterday was another one of those days. But I also realised that you have to start the day right, Dehan's first nap of the day is anytime between 07:00 and 08:00 for about 60 minutes, if he misses that one the whole day becomes a mess.
Get the book, it's so worth it!