G.B.
The reason Lego's have 4 and up on them is because many child care facilities can't allow them due to state regulations regarding choke hazards and any toy fitting through the tube of the toilet paper. Lego's fall through, hot wheels cars fall through, marbles, raisins, grapes, some puzzle pieces, so many different toys they just can't be named/listed here.
If he is ready for them and doesn't chew on toys then let him have them for special time play for dad and son. They can be put up except when dad and son are going to play together.