"Was going to (do)" is correct. "I am/You are/He is/We are/They are going to (do)" (the periphrastic future) shows a planned future action relative to the present. Ex: Joe turned down our party invitation, because he is going to help [he plans to help] his uncle move to a new appartment. If you have the well-known book English Grammar in Use by Murphy, consult Unit 20. "I was/You were/He was/We were/They were going to (do)" shows a planned future action relative to a moment in the past. Ex: Joe turned down our party invitation, because he was going to help [he planned to help] his uncle move to a new appartment. Consult Unit 47 of English Grammar in Use. "Supposed to (do)" has multiple meanings. It can mean "people say." Ex: The restaurant is supposed to be good. Also, "supposed to (do)" can mean a required, suggested or planned action. Ex: Joe is supposed to help [Joe plans to help/Joe has agreed to help] his uncle move to a new appartment on Saturday. However, Joe is in bed with a fever, so it's unlikely that he will be well enough by Saturday to help his uncle move. Consult Unit 45 of English Grammar in Use.

"I was going to" is excellent. "Was going to", "was supposed to", and "was due to" have slightly different meanings. The first is the most general and can be used in all situations. "Supposed" suggests intention or obligation. "Due" suggests scheduling.

'Was going to' is perfectly correct.

All three of the ways you're expressing the first clause are grammatically correct, but "due to" is extremely formal in tone. "Was going to" would be more appropriate if he let you know that he wouldn't be coming. Otherwise, "was supposed to" is the better choice.