The nuance differs.
1) The activity was tedious.
"Tedious" is an innate property of the activity as described.
2) The activity was tedious for him.
"Tedious" is innately linked to his performance of the activity. Depending on the context, it may imply the share of the activity that fell to him was tedious, or that he lacked the ability to perform the activity easily.
3) The activity was tedious to him.
Here, "to him" reflects his feelings about the activity. Others might or might not agree, but to him the activity seems tedious.
Typically in English one normally sees "it was tedious for him" but "it seemed tedious to him", which makes the distinction clearer - the second case makes the subjectivity of the judgement explicit.
Compare "Going outdoors was tedious for him" vs "Going outdoors was tedious to him". In the first instance, replacing "was" with "seemed" changes the nuance of the sentence, but in the second instance it does not.
I would suggest similar syntax applies to a whole range of adjectives that imply judgement e.g. "Being absent from his post was strange for him" and "Being absent from his post was strange to him".
Given that you are describing the activity as actually tedious rather than expressing the feelings of others, "for" is correct.