C., you have great advices here from mamas !
I would say:
try to eliminate multiple events around him
when you want him to focus:
if homework, then no TV or music on the background;
if playing with legos or cars on the carpet, then no TV (again)
if TV, then no eating in front of TV,
when you help him, try to point on some interesting things that happen:
you walk: look for birds,
you drive, ask him to count all the red cars,
and you count all the blue cars, and see whether you met more red or blue cars...
also a great game on teaching to be attentive is moving objects
you choose one room, and ask him to look VERAY ATTENTIVELY at everything aorund, what is where, and how positioned.
then ask him to go to another room for a minute,
and move something: a pillow from one side of the couch to another,
or a vase, to another surface,
or his toy to another corner:
start with larger objects, and later move to little things, like a spoon and such...
then he comes back and needs to find changes: if yes, gets points.
Play it together, and ask him to do the same task for yourself,
then he won't feel that you teach him, but you both have fun.
move out of the room after you looked at things carefully,
and then he needs to move something.
Do not start by REMOVING things completely, this will be too hard for the start, but in future, you ca n develop the game.
this is actually a very ancient Chinese game,and it really helps in becoming more focused...
It is a good time to start caring about this not very big problem,
and in the beginning it will require a lot of your patience,
but with time, it will pay off :)
you will need to constantly remind him what he (or you both) are doing, so you really need to be excited yourself, or it will be dull and boring for both of you:
seek for joy in this teaching-learning process, be creative!
It is a common problem for many kids, as the flow of new-coming information in every moment of the child's life in this developed society is so huge and overwhelming that it seems like the human system just turns off at some point, when it cannot take it any more.
It really helps when you try to make sure that not very many things happen simultaneously around him... tis is the clue, plus exercises that develop concentration abilities.
if he likes doing puzzles, that is also a great game...
I wish you all a lot of joy!