Firstly, your pH is fine. I have attached a link which includes information on pH and notes that a pH of 7.4 - 7.6 is probably ideal.
Axolotls - Requirements & Water Conditions in Captivity
Secondly, from your results I would say that your tank is not cycled. This is essential for the health of your axolotl as it allows the growth of beneficial bacterial on your filter which deals with the toxic waste products. I have attached some links dealing with cycling, and I would suggest that you keep your axolotl in the container with daily water changes using dechlorinated water while your tank cycles.
Caudata Culture Articles - Cycling
FISHLESS CYCLING - HOW TO DO IT | Think Fish Tropical Aquarium Forum
Thirdly, as I mentioned before I am not experienced with very young axolotls, so I spent a bit of time looking at pictures of other youngsters for comparison. Again, please see attached link.
Silly Axolotls
Although the general rule for axolotls is that they should have a body as wide as their head, this is not true for very young axolotls. Although your little one does seem slim, I'd suggest that as long as he is eating and pooping regularly then he is probably fine, and will start to fill out as he grows.
Lastly, your axolotl will be fine at 14C, so it would be best to keep him at that temperature, rather than trying to warm the water, as the fluctuating temperature may be a little stressful.
Axolotls - Housing in Captivity
I hope that one of the more experienced keepers will comment to either confirm all of this, or provide you with more specific information on your little one.
If you have any further questions please do not hesitate to ask. We've all been through tank cycling, worrying about our axolotls, etc. so you are in good company on this forum.
Best of luck and please keep us posted on your progress. :happy: