He should be trimmed every time mom is trimmed - every 6 weeks. Likely all he will need is a light rasp ove rthe heel and toe but its good training too. Don't wait longer "because he is a baby" or his feet "look ok" All foals that are not trimmed generally develop under run heels and long toes whether you can tell or not. This does not allow for proper foot development as the baby grows. This is a critical age and long toes and low heels now may seriously inhibit his developing foot.
Also, turn him out over varied terrain. Hard and uneven is important. Soft lush pasture only is not a benefit to young feet.
The first 5 years of your horses life is the most important to developing a healthy sound foot for the rest of his life.