We suggest you start with LetterSounds 1 Pro, but use the 'custom activity' option to create activities with just a few letters (try 's, a, t, p, i, n', or 2-3 of those letters to begin with) and reduce the 'end score' to 2 to get started. Then gradually increase the 'end score' and add more letters as your son gets more confident. As he gets better at it, increase the 'start score' until he can identify her letters without the mnemonics. If he gets to about 80% accuracy with his letter recognition, start him off with Blending Sounds 1 Pro. You can find more info on LetterSounds 1 Pro here:
..and there are videos of a 3 year old using the 2 apps described above here:
The trick is to become very familiar with the apps yourself before trying them with students - especially with how to customise them for your child's ability level. Also, keep sessions very short (5 mins or so), and fun! Never 'push' young children when teaching them anything - if they are finding it tricky, reduce the number of tiles and / or the 'end score'. Remember that this is a very young age to start learning to read! It MUST be fun!