Normally "little" would be used in this case, which works better with "baby" than with "son". You can also say "I have a young son" and "Your boy is too young to walk". Saying "young" baby is not the best, because a "baby" is always young, so "little baby" is better in this case.

In the first two examples, you could use either word and it would mean the same thing. In the second example, the first is correct, but it's because the word "little" is being used to refer to age.

there are implicit approximate ages for words phases of childhood - child (3 or greater), toddler ( <3, already standing), baby (< 2, not walking)

little is correct in each case. small is somewhat ambiguous in that it may refer to size, 'my little boy is quite big/small for his age'.

I have the same problem so I prefer to use young or younger for children :)