I would dump him fast. In a nutshell, that person either doesn't care for you too much, is trying to hint that he really doesn't want the relationship, or he has someone else. There is a basic lack of respect for you there perhaps because he no longer finds your company as attractive and stimulating as he would wish, hence the constant excuse. People do get busy from time to time but when someone really likes you, they are excited by it and actually make the time to keep connected with you.
Many people find it hard to break off relationships; they also change their minds about pursuing someone but lack the courage to say so. Instead they use work as a handy excuse to cool the friendship. The minute the excuses become regular, you should look elsewhere because that is not saying much about how valued you are for him to even make the time to speak to you. He is probably using the time to speak to someone else. You will always be able to tell if someone is really interested in you because they will actually SHOW it by being keen to call you.