The fact that he is looking on instagram or google, instead of on actual inappropriate content sites is probably a good thing. It likely means he hasn't decided to go onto full fledged inappropriate content sites, which eventually most wind up doing in order to find more novel material.
Your husband likely does feel guilty about it, but is going to have a hard time expressing that to you, because he knows he is hurting you, but still gets drawn to it.
If he is a Christian I would start by discussing with him the Scriptures from my first post and challenge him that it is not just an innocent activity, it is adultery, and a sin against the Spirit and he needs to turn away from it.
You may also want to read the following:
Gal 5:16 But I say, walk by the Spirit, and you will not gratify the desires of the flesh.
Gal 5:17 For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do.
Gal 5:18 But if you are led by the Spirit, you are not under the law.
Gal 5:19 Now the works of the flesh are evident: sexual immorality, impurity, sensuality,
Gal 5:20 idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions,
Gal 5:21 envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God.
Gal 5:22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness,
Gal 5:23 gentleness, self-control; against such things there is no law.
Gal 5:24 And those who belong to Christ Jesus have crucified the flesh with its passions and desires.
Gal 5:25 If we live by the Spirit, let us also keep in step with the Spirit.
While your quite natural response is to feel revulsion at what your husband did and is doing, and you do need to actually communicate that to him to help motivate him, be sure also to keep in mind that the marriage has been good to this point. It can be again. He needs to break this off for his own spiritual life and for your marriage.
If you really want to see change there will have to be a time where you work together to build trust and he is willing to be accountable to you in this regard. This may mean allowing you to check his phone, etc.
On the other hand, you will want to take more time together to to do activities, go on dates, etc. to rebuild the emotional aspects of the marriage.
This may feel forced at first if you are deeply hurt, but you can reconnect especially as he starts to show actual change in behavior.