like the title says, is there a way to manually create a glossy/metallic lightning effect on certain part of the object ? I am using Adobe Photoshop CS6.
So, since you have the metal texture on a lot of your image, it's going to be difficult (and probably not desirable) to get a "plasticy" gloss on your figure.
However I can recommend a couple of simple techniques to amp up the reflectiveness that you've already got going on.
First let's look again at your original image.
Now, let's start off by using the levels adjustment so that we can amp up the whites and create more contrast.
Next, if you want to go even further, take the pen tool and outline sections of your image that you want to be even more reflective. Go ahead and right click to make that area a selection. (I didn't feather because I prefer to blend manually for stuff like this later)
Now, add a gradient from white to transparent following the already present direction of light on your image.
Next, head over the the layers panel and set the gradient layer to overlay with about 20% opacity or so (really, whatever looks best to you).
Repeat the pen tool steps for each highlight on your image that you want to amplify. For this example, it did one other small section on the dude's biscep.
Finally, blend the edges of your new gradient sections using the blur tool.
Let me know if you have any questions and/or if this method works for you!
This is my suggestion. In photoshop use new layers on top of your originl drawing and add some small reflections but without blur or soft edges.
Use really well defined edges with whites and lighter colours. You could also use blending modes on those layers such as "Overlay" and "Screen" for zones where light is reflected.
To add even more depth you can also use some dark shade zones again without gradients of soft brushes. For these dark zones use blending modes like Multiply and work with the layer's transparency.
The more defined your edges are the glossier the look. :)