Have you tried keeping him near you while you work and giving him some things that he can entertain himself with?  For example, if you are in the kitchen, open a cupboard and let him play with the pots and spoons.  If you are on the computer, see if you have an old keyboard that he can bang on.  If you are trying to read, set him up next to you with his own stack of board books.  Try to rotate the toys he plays with so he will not get bored of them.  Also, consider household items that may be of interest to him.  Your son may need to be shown how to play with a toy before you leave him to do it himself.  Some babies need to be taught to self-entertain.  As they get older, they will be able to do this for longer periods of time.