It may or may not be "normal" for mastiffs to be skinny - but thinner is definitely healthier. The slower they grow, the better... so being able to feel - or even see - a few ribs is preferred. If the hips are poking through, and don't have some padding, that might be a bit underweight, though.
We transitioned Denna to raw when she was 6 months old, so almost the same age as your pup. We had a few issues with 'loose poops' - mainly from feeding too much at one meal and/or introducing new/different proteins too soon.
A good source of "how to" (at least, the way we tried to do it) is available here:
Raw Fed Dogs - Natural Prey Model Rawfeeding Diet
The best recommendations I can give you are:
1. go SLOW - one protein (i.e. chicken), no organs for at least two weeks... then add in new stuff in small portions. You'll notice on that link, liver is not introduced until week 11!
2. if you start with chicken, buy whole chickens, whack them into 4 pieces, and feed one piece per meal the first day (probably be <1lb per meal), then increase from there (assuming poops stay solid) until you reach your desired lb/meal.
3. we started with 2 meals a day (and still feed 2 meals a day) - keeping the meals smaller I think helps the transition, too. Gives the system time to shift gears and get caught up with the new digestive process - which includes higher stomach acid production.
4. balance nutrition Over Time. I normally try to balance out each week, but some people aim to balance monthly. That means - for me - each week I'll pull enough stuff out of the freezer to get my 80/10/10 (meat/bone/organ) ratios set, then just feed random items from the bucket in the fridge for meals during the week.
5. veggies - not "necessary"... but I add them in often. They add bulk to the poops (hint: they're not fully digested) - which I'm hoping helps Denna keep her anal glands clear. She also doesn't do well with lots of bone, and will get constipated, and veggies help provide some stool-softening for her. Our regular veggies include squash, carrots and broccoli. I use a frozen mix, hoping that the freezing process has broken the cell walls some, allowing some nutrients to be bio-available to the pup. If you use raw, you'll often notice them coming out in the poop in the same form they went in.
6. fruits - also not "necessary" and not very digestible... but Denna LOVES her fruit. She's still working on getting the last raspberries off the bushes in the yard. She also ate waaaaay too many fallen apples off the ground when staying with my MIL one weekend... and got the runs.... so, not something to over-do, for sure!!
Enjoy! It's an adventure, but I find it to be totally worth the effort.