Eat about 2-4x what you are eating (slowly ratchet up the calories every week) and lift heavy things. You may also want to investigate adding dairy (milk, yogurt, always whole-fat, strongly recommend grass-fed and non-homogenized, preferably raw as well) into your diet. You might want to eat some sweet potatoes, especially post-workout, and possibly reduce your fruit intake if that allows you to pack in more calorie-dense carbs via sweet potatoes.

Take a look at a previous PaleoHacks question about weight gain for more tips, and also Robb Wolf's mass gain experiment.

Robb has elsewhere recommended eating a TON most days, with a "base calorie" day every week or so (meaning, eat only as much as you currently eat) and a base calorie week every 4-5 weeks or something like that. The idea is to give your metabolism a bit of recovery time and to maintain insulin sensitivity.

I second what pjnoir says, but take it to the extreme: One Gallon of Milk a Day (OGOMAD)

Seen on forums like startingstrength and stronglifts, this is the way to get big if you are serious...you might gain a little fat, but you can't gain all lean

I don't buy 'cant gain weight on paleo' arguments, just eat more and try a heavy barbell routine like those at the aforementioned forums: startingstrength.com and stronglifts.com

I agree with Jae's answer. Heavy weight training is important. Presumably you want to gain lean body mass, not fat, so you need to stimulate the muscles to grow.

Lean pork, canned tuna, salad, vegetables--I feel depleted just reading your diet! I mean that in a friendly way. ;)

Aside from the bacon and eggs at breakfast (and a bit of olive oil) you are not getting much fat in your diet. People debate between low carbs and low fat, but restricting both is a sure fire way to be too skinny. Switched to canned salmon (I'm assuming cost is the reason for not going with fresh fish) and fattier cuts of meat. If you can find meat that is raised naturally, the fatter cuts the better.

I also agree about more carbs from starchy tubers, and grouping them around your workouts (60-90 minutes before and after). Carbs are protein sparing i.e. your body can use the protein to build muscle, rather than burning it for energy.

Good luck! (Let me know if you need some tips for a workout plan.)

If you're eating this way because of skin issues, I wouldn't be drinking any milk if I were you. Just take a look at your menu, it's very low calorie and low fat. Lean pork/canned tuna? Mehhh. Add some fat! Switching to the eggs for breakfast is a good idea, but do the same for your other meals. More fat, preferably animal fat (aka not too much dairy beyond butter) because of your skin issues.

The only tuber I eat and not that often are WHITE skinned sweet potatoes with the skins, but to gain weight that is more muscle that body flab, milk (non-paleo I know) is the best way. Add DEADLIFTS to your weight routine and you will pack on some serious muscle. With milk, whey is not even needed. When you reach the weight you want, cut back on the diary if you want. Meat protein is important but you will not gain mass with it alone. Body builders eat massive amounts of calories to produce an ANABOLIC condition ( the same the little pill produces with the meals) for growth. Many become diabetic at the end of their careers. The beauty of Paleo (no diary) and Primal/PaNu (diary) is that the body finds its ideal body weight. Good Luck.

Everyone is advising increased calories. Of course, your metabolism will speed up as a response. You may want to experiment with some amount of fasting (weekly?) to try to keep your metabolism in check.

I'm a little confused -- you've been on a low carb diet for two months, presumably, to lose weight. Why are you now looking to gain weight? I'm just curious as to your motivation.

Do I absolutely have to eat starchy tubers ? I have a hate/hate relationship with most.

I'm a slender fellow, and started eating Paleo about a year ago. The trouble is that I already had difficulty gaining weight, and found it even more difficult while eating Paleo. Eventually, I started experimenting with the one meal I kept neglecting... breakfast. I settled on what I call the "Tuna Egg-splosion", and it's worked pretty damn well for me.

http://www.unfinishedman.com/tuna-egg-splosion-breakfast-weight-gain-meal/

I managed to gain about 1lb per week and keep it on, which - while not super rapid - has been good enough for me.