Throwing money at equipment is probably not the right answer here. I'm not trying to be snide, but it's more about what you do with the equipment, than what equipment you have. I've met several new parents recently who all bought expensive new camera kit, but remain puzzled as to why their photos remain a bit dull.
A 50mm 1.8 lens is pretty much ideal for portraits, especially as you're mounting it on a DLSR (it turns into a 75mm lens, perfect for portraiture). If you can manual focus well enough, set it at a wide apeture and blast away using natural light. If you're manual focussing is a bit sketchy, try using a smaller apeture (smaller apeture = *higher* apeture number, like 5.6 or 8) which will bring more of the shot into focus. But you'd be better off just practising with manual focus- it should come quite naturally after a while.
If you've been worried about flash: use as much natural light as possible. Ideally it would be diffused- through curtains or reflected off a nearby wall. Direct sunlight tends to lead to harsher looking photos. If it's dark, then you can use a flash diffuser to bounce light off a wall or ceiling- this leads to much more even, softer lighting and also avoids the problem of blasting a flash straight into your baby's eyes. This is a diffuser that clips onto your existing camera flash: light scoop digital photography lighting built-in flash Nikon Fuji Canon Pentax
Beyond the technical stuff, good baby photos are down to technique. Watch your framing- sticking your baby's head smack bang in the middle of a photo tends to be a bit boring. Instead, try framing the baby off to the side a bit (rule of thirds, quite fundamental in photography- AIRC - Adorama Imaging Resource Center: Portrait composition rules, and when to break them). Also, be patient and be prepared to look through the viewfinder for a long time before your baby comes up with an interesting expression. And take photos of him when he's distracted by other things besides you and a camera.
Lastly, remember you've got a digital camera with no film costs. You can take hundreds of photos, then look at them later and decide which ones are good. More importantly, you should start to recognise what *makes* those ones good and learn from them.